Basic Obstetric Ultrasound 

Updated: May 04, 2021
Author: Christine Kansky, MD; Chief Editor: Carl V Smith, MD 

Overview

Background

The basic obstetric ultrasound examination provides an accurate and safe clinical assessment of the gravid uterus throughout a woman’s pregnancy including characterizing pregnancy location, identifying the number of embryos present, and aiding in the prenatal diagnosis of fetal anomalies. In 2013, the American Institute of Ultrasound in Medicine (AIUM), in conjunction with the American College of Radiology (ACR) and the American College of Obstetricians and Gynecologists (ACOG), released updated Practice Guidelines for Performance of Obstetric Ultrasound Examinations. These guidelines describe the indications and key elements of 4 major types of obstetric ultrasounds, specifically the first trimester ultrasound, standard second or third trimester ultrasound, and limited and specialized ultrasound examinations.[1] Further details regarding specific information gathered in each type of ultrasound exam is described below.

Also see the article Targeted Obstetric Ultrasound.

Indications

First trimester ultrasound

The first ­trimester basic ultrasound is typically performed to confirm a viable intrauterine pregnancy. The exam may be performed either trans-abdominally or trans-vaginally. It is ideally performed before 13 weeks and 6 days of gestation. Ultrasound examination at this time aids in the clinical assessment of pelvic pain and/or vaginal bleeding in the setting of an early pregnancy because it can diagnose an extrauterine pregnancy or an abnormal pregnancy, such as a hydatidiform molar pregnancy, an anembryonic gestation or an incomplete versus complete abortion.

A definitive diagnosis of an intrauterine pregnancy can be made when a gestational sac containing a yolk sac is visualized within the uterine cavity. Without visualization of a yolk sac (or signs of a further developed pregnancy such as an embryo), the location of the pregnancy cannot be certain and further evaluation is warranted. In some cases where a pregnancy test is positive but there is no clear intrauterine pregnancy or extrauterine findings concerning for an ectopic pregnancy such as an adnexal mass on ultrasound, a patient may have a “pregnancy of unknown location.” It is important to consider the clinical context of a patient without a documented intrauterine pregnancy to guide further management. This includes the patient’s symptoms (pelvic pain, vaginal bleeding), serial serum beta human chorionic gonadotropin levels (bHCG), and pelvic exam findings.

Additionally, a first trimester ultrasound examination is useful to diagnose an “early pregnancy loss” which is defined by American College of Obstetricians and Gynecologists as a nonviable, intrauterine pregnancy with either an empty gestational sac, or a gestational sac containing an embryo or fetus without cardiac activity within the first 12 6/7 weeks of gestation.[2] The Society of Radiologists in Ultrasound Multispecialty Panel on Early First Trimester Diagnosis of Miscarriage and Exclusion of a Viable Intrauterine Pregnancy have published conservative guidelines to aid with clinical judgment in the diagnosis of an abnormal intrauterine pregnancy. Diagnostic findings of an early pregnancy loss include: 1.) Crown-rump length of 7 mm or greater and no heartbeat, 2.) Mean sac diameter of 25 mm or greater and no embryo, 3.) the absence of cardiac activity in an embryo 2 weeks or more after a scan that showed a gestational sac without a yolk sac, and/or 4.) the absence of cardiac activity in an embryo 11 days or more after a scan that showed a gestational sac with a yolk sac.[3]

Cardiac activity of an embryo is documented using 2-dimensional video clip or M-mode imaging.[1] If no cardiac motion is seen on transvaginal ultrasound in an embryo less than 7 mm, a subsequent ultrasound in 1-2 weeks should be performed to assess for cardiac activity.[4] Additionally, if the patient’s clinical presentation suggests a miscarriage (i.e heavy vaginal bleeding) but she is stable for expectant management, a follow-up ultrasound performed 7-14 days after initial presentation to assess for interval changes and viability is also appropriate management.[2]

In the setting of a confirmed viable intrauterine pregnancy, the first ­trimester ultrasound is utilized to provide an accurate gestational age assessment. When only a gestational sac and yolk sac are visualized, the mean gestational sac diameter may be used to estimate gestational age (Mean sac diameter (mm) + 30 = gestational age in days). However, if an embryo is visualized then a crown-rump length (CRL) of the fetus should be used to determine an estimated due date because it is the most accurate measurement for establishing gestational age.[4] An embryo should be visible by transvaginal ultrasonography with a mean gestational sac diameter of 25 mm or greater. The crown rump length is the maximum length of the infant from cranium to caudal rump in a longitudinal plane.[4, 5]  Measurements of the CRL are more accurate the earlier the first trimester ultrasound is performed.  If the CRL measurement is greater than or equal to 84 mm (which corresponds to a gestational age of 14 and 0/7 weeks), second-trimester biometric parameters should be used for calculating the gestational age.[5] A reliable formula to calculate gestational age based on CRL is as follows: CRL (mm) + 42 days (+/- 3 days) = gestational age (days).[6]  

In 2014, ACOG published a standardized approach for calculating a patient’s anticipated due date using both ultrasound estimates and menstrual history, specifically the patient’s first day of the last menstrual period.[5] If the patient is unsure of her last menstrual period (LMP) or has a history of irregular menstrual cycles, dating should be calculated based on ultrasound measurements. In general, ultrasound dating is used when the discrepancy between menstrual dating and ultrasound dating is greater than the precision of ultrasonography.[4] First trimester calculations are more precise compared to later gestational ages. Before 14 0/7 weeks gestation, the mean crown-rump length calculated has a precision of 5-7 days.[4] Therefore, before 9 0/7 weeks gestation, the estimated due date should correspond to ultrasound measurements when there is more than a 5 day discrepancy between menstrual dating and ultrasound dating.[4, 5] Similarly, if the ultrasound dating between 9 0/7 weeks of gestation and 13 6/7 weeks gestation has more than a 7 day discrepancy from the menstrual dating, ultrasound measurements should be used to assign estimated due date.[4, 5] In the second and third trimester, larger discrepancies reflect less precise measurements based on biometric parameters (see Table 1).

Table 1: American College of Obstetricians and Gynecologists’ Guidelines for Redating a pregnancy based on ultrasonography rather than menstrual dating.[5]

Table. (Open Table in a new window)

Gestational age in weeks

(by LMP)

Discrepancy between menstrual dating and ultrasound estimates  

8 6/7 or less

> 5 days

9 0/7 to 13 6/7 

> 7 days

14 0/7 to 15 6/7 

> 7 days

16 0/7 to 21 6/7

 > 10 days

22 0/7 to 27 6/7

> 14 days

28 0/7 and beyond

> 21 days

In the setting of multifetal gestations, amnionicity and chorionicity should be documented. In the setting of desired genetic testing, nuchal translucency measurement aids in the screening assessment for fetal aneuploidy in conjunction with biomarkers. A first trimester ultrasound is also useful for the evaluation of maternal anatomy including assessment of the uterus, cervix and adnexal structures. The presence of adnexal masses, ovarian cysts, and/or leiomyomas should be documented and followed throughout pregnancy.

Second and third trimester ultrasound

Second or third trimester ultrasound examinations use fetal biometry to assess fetal growth and also can provide detailed information on fetal anatomy. A standard obstetric ultrasound examination also may include an evaluation of fetal presentation(s), amniotic fluid volume, cardiac activity, and placentation.

After the first trimester, fetal biometry specifically measures the fetus’ biparietal diameter, head circumferences, abdominal circumference or average abdominal diameter and femoral diaphysis length. Fetal biometry may be utilized to establish an estimated due date for a pregnancy if no prior ultrasound measurement of the embryo was done. However the most accurate gestational age assessment is based on crown rump length measurement and the variability of gestational age estimations increases throughout pregnancy. For this reason, the earliest available ultrasound should always be used to assign an estimated due date and any significant discrepancies between gestational age and fetal measurements on subsequent ultrasounds should raise suspicion for growth abnormalities. The approximate error in fetal weight prediction methods is approximately 15% and is influenced by patient body habitus, weight range of fetus, technical factors such as machine quality and experience of ultrasonographer.[4]   In the third trimester, the femur length is the best single biometric measurement of gestational age.[4]

Assessment of fetal anomalies, also known as a fetal anatomic survey, should be performed after 18 weeks gestational age and ideally performed between 18-20 weeks gestational age.[4] Although it may be possible to document anatomic structures before this time, the size, position, and movement of a fetus may limit a comprehensive examination of structures and therefore require repeat ultrasound examinations.[4] Additionally if a fetal anomaly is detected in this preferred gestational age window (18-20 weeks), termination of pregnancy may still be an option for the patient. The basic fetal anatomic examination includes assessment of the following structures: lateral cerebral ventricles, choroid plexus, midline falx, cavum septi pellucidi, cerebellum, cistern magna, upper lip, four-chamber view of heart as well as left and right ventricular outflow tracts, size and location of stomach, urinary bladder and ureters, spinal anatomy, extremities, and gender. A more detailed anatomic survey may be indicated depending on the risk and concern for aneuploidy. The placenta should also be further characterized at this time, specifically noting its location and proximity to the internal cervical os as well as the number of vessels and insertion site of the umbilical cord [1,4].[1, 4]  

Additionally, the second and third trimester basic ultrasound may also be used to diagnose or to monitor maternal anatomical problems, most notably cervical length in the setting of risk factors for preterm birth or cervical insufficiency. Ultrasound monitoring of fibroids and/or ovarian cysts is also important, but this is limited at later gestational ages due to the size of the gravid uterus.

Limited and specialized ultrasound exam

A limited or specialized ultrasound may be performed at any gestational age and is typically used to evaluate a specific clinical concern during prenatal care. Some examples include assessment of cardiac activity when fetal heart tones are undetectable with external fetal monitoring devices to rule out fetal demise, notation of fetal presentation in setting of anticipated external cephalic version or to determine mode of delivery, and calculation of estimated fetal weight and amniotic fluid in setting of comorbidities that may predispose infants to growth abnormalities (eg chronic hypertension, pre-eclampsia, diabetes, multifetal gestation).  

Fetal growth evaluations are typically performed at 3-4 week intervals and usually include an assessment of amniotic fluid as well.[4] There are two main techniques to measure amniotic fluid in the second or third trimester, specifically single deepest pocket (SDP) or the amniotic fluid index (AFI). The SDP technique records the single largest vertical pocket of amniotic fluid without evidence of umbilical cord or fetal parts visualized in utero. The AFI technique is the summative measurement of the single deepest vertical pocket of fluid without evidence of cord or fetal parts noted in all four quadrants of the uterus. Oligohydramnios, or significantly low amniotic fluid, is defined as an AFI less than 5 cm or a maximum vertical pocket less than 2 cm. Alternatively, polyhydramnios which is a term to describe an abnormally large amount of fluid, is defined by an AFI greater than 24 cm or a single deepest vertical pocket (SDP) greater than 8 cm.[4] In multifetal gestations, fluid evaluation should be performed using the single deepest pocket technique. Comparison of measurement techniques show that measurement of the single deepest pocket leads to fewer interventions (such as induction of labor for oligohydramnios) with no increase in poor perinatal outcomes.[4]

Similar to a limited ultrasound exam, a detailed ultrasound examination is a supplemental tool to aid in the management of prenatal care in the setting of concern for fetal well-being due patient history, genetic screening abnormalities, or results of prior ultrasound exams. For example, The CDC and ACOG recommend that pregnant women who live in or have traveled to areas with ongoing Zika virus exposure should undergo Zika virus serologic testing and fetal ultrasonography to screen for microcephaly or intracranial calcifications as early as 3-­4 weeks after symptoms or exposure.[7, 8] . However, the CDC warned that fetal ultrasounds might not detect abnormalities until late second or early third trimester of pregnancy.[7, 9, 10] Other examples of specialized ultrasound exams include fetal surveillance with a biophysical profile, fetal Doppler ultrasound for assessment of placental insufficiency, and a more detailed anatomy scan or fetal echocardiography in setting of concern for fetal anomaly.

Safety

The prenatal ultrasound examination has been proven safe to both mothers and fetuses. As with any clinical test or medical intervention, a risk ­to ­benefit analysis of the test should be considered and should only be performed when there is a medical/obstetric indication or clinical concern. Additionally, if a diagnostic ultrasound is required for patient care it should be done under the “as low as reasonably achievable principle” (ALARA principle) due to possible risks associated to the physical effects from the exam including mechanical vibrations or increase in temperature under exam conditions as well as unknown risks of ultrasound energy to fetus not yet documented in the literature.[1, 4] Some ultrasonographic modalities, such as Doppler, deliver more energy to the area of interest and the use of those modalities should be reserved for specific clinical questions and an attempt made to limit their duration of use.

Contraindications

In 1994 the Federal Drug Administration reported concern about the misuse of diagnostic ultrasound exams and equipment for non-medical purposes, specifically noting that the promotion, selling, or leasing of ultrasound equipment for “keepsake” fetal videos without a physician’s order may be in violation of local or state laws and regulations.[11]

Technical Considerations

Obstetric ultrasound examination requires real-time two-dimensional imaging via a trans-abdominal or trans-vaginal approach in order to adequately assess pregnancy viability through cardiac activity and fetal movement. There is currently no clinical evidence suggesting a clear advantage of three-dimensional imaging in prenatal diagnosis.[4]

The ultrasound transducer frequency must be selected to balance optimal beam penetration versus resolution, and therefore may vary based on a patient’s body habitus. For example, a lower-frequency transducer is beneficial in obese patients to allow for increased penetration and better imaging. Modern equipment typically includes a trans-abdominal transducer with 3 to 5 MHz frequency and a trans-vaginal transducer with 5-10 MHz frequency.[4]

Modern ultrasound equipment has boundaries set by the manufacturer, limiting the fetal exposure to energy generated by the equipment. As with any medical equipment, adequate care and maintenance should be performed as per manufacturer recommendations. Additionally, personnel involved with the use of ultrasound equipment should have appropriate training. Some ultrasound suites have quality ­assurance programs to evaluate performance of personnel and the ultrasound unit. The AIUM has published specific training guidelines for physicians who perform and interpret obstetric ultrasound which includes the following minimum requirements to demonstrate a strong knowledge base, technical skill and competency.[1, 12] Specifically for graduates of residency or fellowship programs to gain proficiency, the physician must perform at least 300 diagnostic ultrasounds. Additionally, a minimum of 170 annual diagnostic obstetric ultrasound examinations is recommended to maintain the technical skills required for competency.[12]

Procedure planning

Minimal preparation is required for a trans-abdominal or trans-vaginal ultrasound. A fasting state is not required, in contrast to other ultrasound studies (eg, gallbladder ultrasonography). Some practitioners advise their patients to arrive to the ultrasound suite with a full bladder, but there is no consensus regarding this recommendation, especially for an obstetric ultrasound performed after 18 weeks’ gestation. If a trans-vaginal approach is to be used, the patient is asked to void just before the study to empty her bladder. This minimizes discomfort and collapses the bladder for better visualization of pelvic organs.

Ultrasound transducers require proper cleaning before and after each patient’s use to avoid risk of microbial transmission leading to infection. Transabdominal ultrasound transducers are typically cleansed with disposable antiseptic wipes and the clean transducer may be applied directly to the patient’s skin. Conversely trans-vaginal ultrasound transducers require more extensive cleaning and sterilization. Specifically trans-vaginal transducers should be covered with a single-use disposal cover during the patient exam. After the exam is completed, cleaning steps include removal of the disposal cover, cleansing with running water or a damp cloth to remove residual gel or debris from probe, followed by high-level disinfection chemical agents in accordance with FDA recommended guidelines.[4]

 

Periprocedural Care

Patient Education & Consent

Elements of informed consent

Most practitioners obtain verbal consent from patients. Ideally, this process should be initiated by the practitioner who requests the study. Risks, benefits, expectations, and limitations of the study should be presented. Some of the limitations relate to the inability of an ultrasound to detect all fetal anomalies, as well as the impossibility to rule out the risk of false-negative ultrasound results.

Additional consent is requested for transvaginal studies.

Equipment

The physics and principles involving ultrasound are complex, and the curious reader is encouraged to refer to other references for an in-depth discussion of those.

The most important part of the ultrasound machine is the transducer, the tool that makes contact with the patient’s skin. It is equipped with piezoelectric crystals that can generate a sound pulse when excited by an electric current. That sound is released at a specific frequency, travels through human tissue, and interacts with it based on physical properties of the tissue. Sound is absorbed by the tissue, and reflected back to the transducer, where the piezoelectric crystals change that message to electrical impulses that are sent to the processor for interpretation. The processing unit interprets the signals and creates the image we see on the screen.

Patient Preparation

Anesthesia

For all diagnostic ultrasound studies and most, if not all, ultrasound-assisted procedures (eg, amniocentesis), anesthesia is not required.

Positioning

For abdominal studies, the patient is in a supine position for most of the study, with the abdomen uncovered to provide skin contact with the ultrasound transducer. She may keep their clothes on. A coupling gel is used between the transducer and the skin to reduce acoustic impedance from environmental air. This gel may be warmed for patient comfort.

For transvaginal studies, the patient is to remove clothing from the lower part of her body and to lie in a dorsal lithotomy position. Regular stirrups may be used to assist with positioning, or the patient may be asked to assume a frog-leg position. The pelvic area is covered by a blanket for the patient’s comfort. The transvaginal probe is covered with a coupling gel, and a vinyl or latex cover is then placed around the transvaginal ultrasound probe. Another layer of gel is applied around the covered probe to assist in the vaginal insertion and to reduce the acoustic impedance.

Some patients may feel that they are more “in control” of a transvaginal examination if they insert the vaginal probe themselves. Personnel involved in performing transvaginal studies need to be sensitive to patient’s requests.[13]

 

Technique

Approach Considerations

An abdominal approach is acceptable for most first- and second-trimester ultrasound examinations.

In some cases, such as those involving maternal obesity, abdominal scars due to prior surgeries, close evaluation of the cervical length during the second trimester, or gestation less than 8 weeks, a transvaginal or translabial approach may be used. The ultrasound probe used with this approach is smaller to gain access through the vagina. In addition, the sound frequency that the probe emits is higher, as the target is closer to the probe.

In general, nuchal translucency evaluation during the first trimester may be accomplished with the transabdominal approach.

First-Trimester Ultrasound

Approach to a first-trimester ultrasound

The first-trimester ultrasound examination is used mainly to confirm intrauterine pregnancy, to confirm dating, and to assess nuchal translucency. The uterus, cervix, and adnexa should be evaluated for location of a gestational sac. If a gestational sac is seen, the presence or absence of yolk sac should be reported.

Discriminatory levels of human chorionic gonadotropin (hCG) levels and ultrasound findings have been reported. Those are quantitative levels of the pregnancy hormone beta–human chorionic gonadotropin (b-hCG) and expected ultrasound findings in a viable pregnancy. They may help predict the viability of intrauterine pregnancies with uncertain viability (IPUV).

Table 1. Mean Gestational Sac Sizes at Which a Yolk Sac and an Embryo Should Be Visible (Open Table in a new window)

Visible Feature

Mean Transvaginal Gestational Sac Diameter (mm)

Mean Transabdominal Gestational Sac Diameter (mm)

Serum b-hCG level (mIU/mL)

Gestational Age (wk)

Yolk sac visible

8

20

7,200

5-6

Embryo visible

16

25

10,000

>6

The yolk sac can usually be visualized if the gestational sac is approximately 1 cm in diameter. In some cases of embryonic demise, the yolk sac is deflated or irregular.

Fetal number, location, and presence and rate of heart rate should be clearly evaluated and documented. An attempt to determine chorionicity should start during the first-trimester ultrasound. Thick interfaces between gestational sacs suggest dichorionic pregnancies, and thin or absent membranes likely represent monochorionic twins.

A corpus luteum cyst may be observed in the maternal adnexa, usually 3 cm or less in diameter. Both adnexa should be evaluated for the presence of large ovarian cysts such as occurs in ovarian hyperstimulation syndrome, and solid masses such as in ovarian neoplasm. The uterus should also be evaluated for homogeneity, presence, and size and location of fibroids, especially intracavitary.

Embryonic demise

The first-trimester ultrasound presents an opportunity to identify some problematic pregnancies. Early signs of nonviable pregnancies include a gestational sac with an irregular shape or one that is not growing or b-hCG levels obtained from the patient that do not correlate with ultrasonographic findings. Embryonic demise may be diagnosed when the crown-rump length is 6 mm without fetal cardiac activity.

A study found that current guidelines regarding ultrasonographic diagnosis of miscarriage may still be associated with misdiagnoses and should be updated to take into account gestational age.[14, 15]

Ectopic pregnancy

Ectopic pregnancies or heterotopic pregnancies (one fetus inside the uterine cavity while another fetus is implanted outside the uterine cavity) may also be diagnosed. Clinically, ectopic pregnancy may manifest as pelvic pain and/or vaginal bleeding, elevated b-hCG levels, free fluid in the peritoneal cavity usually representing blood, and adnexal findings.

Anencephalia

Severe fetal anomalies can also be diagnosed in the first trimester. Anencephalic fetuses present with an absent cranium on this early ultrasound examination.

First-trimester sonographic markers for aneuploidy

Early tests for aneuploidy include the measurement of the skin swelling behind the fetal neck (the nuchal translucency), along with two biochemical markers, pregnancy-associated plasma protein A (PAPP-A) and free-hCG. First-trimester screening has a detection rate of 87% for aneuploidy. Other anatomical markers for aneuploidy on the first-trimester ultrasound include the absence of nasal bone, increased frontomaxillary angle measurement based on gestational age, the appearance of an objective decrease in blood flow during an atrial contraction (a-wave abnormalities) on ductus venosus Doppler evaluation, and increased tricuspid valve regurgitation.

However, not all of these markers can currently be assessed in the general population.[16]

Second- and Third-Trimester Ultrasound Evaluation

Approach to the second-trimester obstetric ultrasound

The most common indication for a second-trimester ultrasound examination is evaluation of fetal anatomy, usually between 18 and 20 weeks’ gestation. At this point, the embryologic period is well passed, and fetal organs are, for the most part, easily visualized and evaluated. Fetal position, early pregnancy, anterior placenta, multiple pregnancies, uterine fibroids, and maternal obesity may impede a good anatomical evaluation of the fetus at this stage. If the examination is inconclusive because of one of these factors, the patient may be asked to return to finalize the fetal anatomical evaluation at a later date.

Embryo at 12 weeks' gestation. Embryo at 12 weeks' gestation.

In some cases, owing to choice or lack of early access to prenatal care, patients are unable to obtain an early ultrasound examination to confirm the gestational age of the pregnancy, number of fetuses, and chorionicity, which can be determined with a second-trimester ultrasound.

While a first-trimester ultrasound is considered a better tool for gestational dating, the second-trimester ultrasound can be used to determine gestational age. A composite of multiple fetal measures is made and averaged. The common practice is not to override dates based on the calculation of last menstrual period unless there is a difference of 5 days between dates determined by last menstrual period and first-trimester ultrasound or a difference of 10 days between the last menstrual period and second-trimester ultrasound.

Multiple pregnancies

Evaluation of fetal chorionicity when multiples are present may be performed similarly as on the first-trimester ultrasound. If the fetuses are located in different gestational sacs and the membrane dividing the fetuses is thick and has a broad base, the pregnancy is more likely dichorionic. Conversely, if the membrane dividing the twins is thin without a broad base (called a “twin peak” or “delta sign”) or absent, it suggests a monochorionic pregnancy. If the fetuses are different sexes, dichorionic pregnancy is diagnosed.

Placental evaluation

If the patient presents with acute vaginal bleeding, ultrasound may help determine cause. Placenta previa occurs when part or the entire placenta covers the internal cervical os. This diagnosis is important, as it requires cesarean delivery. Abruptio placentae refers to early separation of part or the complete placenta. A hypoechoic area may be seen on the interface between the uterus and the placenta. Vasa previa is rarer and is diagnosed by visualizing placental blood vessels running across the internal cervical os.

Fetal ultrasound testing

Fetal well-being may be evaluated with various measures. A biophysical profile (BPP) is composed of a series of measurements to assess fetal hypoxia. They include amniotic fluid evaluation, breathing movement, gross body movements, and fine movement. A scoring system consisting of either 0 or 2 points is then applied to the fetal evaluation.

Measurement of the fetal umbilical artery (UA) Doppler indicates the resistance that fetal blood flow finds at the placental level. To obtain fetal UA Doppler, the angle of insonation should be as close to zero degrees to the umbilical artery as possible. A comparison of forward blood flow between fetal systole and diastole is obtained. A large systolic-to-diastolic ratio indicates higher resistance of blood flow at the placental level.

Maternal anatomy to be evaluated includes the uterus, adnexa, and cervix.

Fetal structures to be evaluated include the head, face, and neck, including the cerebellum, choroid plexus, cisterna magna, lateral cerebral ventricles, midline falx, cavum septi pellucidi, and upper lip.

Most of the fetal syndromes, aneuploidies, and severe fetal anomalies manifest as fetal CNS anomalies. A normal ultrasound finding does not completely rule out fetal aneuploidies but is reassuring. If the patient had a prior pregnancy complicated by CNS anomalies, the risk of recurrence in a later pregnancy is about 2%.

Fetal growth

Similarly, by following the same parameters measured to obtain gestational age on the second-trimester ultrasound, the pregnancy may be monitored for objective estimation of fetal growth. Clinically, fetal growth is monitored by measuring the fundal height during routine obstetric visits. Fetal measurements obtained to determine fetal age and weight include biparietal diameter, head circumference, abdominal circumference, and femur and humerus length. When growth of fundal height is not as expected in some twin pregnancies or others pregnancies at risk for growth restriction, serial second- and third-trimester ultrasounds may be performed to evaluate adequate fetal growth. Because of variability in ultrasound measurements, a 3-week wait between growth ultrasounds is used to determine fetal growth.

Fetal head circumference should be measured in a plane that includes the cavum septum pellucidum, falx cerebri, and thalamus. To measure the biparietal diameter (BPD), the electronic calipers should be placed from the outside the calvarium proximally to the inside of the calvarium distally, crossing the falx cerebri at a 90° angle.

The same anatomical landmarks used to measure the biparietal diameter are used to measure the fetal head circumference. This measurement is obtained by measuring the outer circumference formed by the calvarium. Head measurements may be helpful to indicate fetal age, microcephaly, or macrocephaly.

Objectively, the cephalic index (CI) is defined as the width of the head divided by the length in a percentage scale. The normal value is between 70 and 80, but an abnormal value does not necessarily indicate pathology. If the cephalic index is more than 80, the fetal head will have a rounded appearance, also called brachycephalia. If the cephalic index is less than 70, the head may appear flat, also known as dolichocephalia. Minor changes in the shape of the fetal head are normal. However, brachycephalia may be seen in trisomy 18, and dolichocephalia may be seen is preterm babies or craniosynostosis.

Fetal anatomy: CNS

The cerebellum is located in the posterior inferior aspect of the brain. It has a large mass of cerebral cortex above and the Pons below. It is divided in two hemispheres. The cerebellar surface folds into itself, increasing the area. Those folds are called lobules. There are 3 major areas: the flocculonodular, anterior, and posterior lobes. This organ has more neurons than the rest of the brain. The functions are related to gait, language, and body tone. Fetal evaluation of the cerebellum includes its presence, shape, and size. During a second-trimester ultrasound, cerebellum is measured by the outer length of the posterior cerebellar hemispheres, at the same plane that the septum cavum pellucidum, cisterna magna, and nuchal fold are seen.

Posterior fossa deformities are seen with Chiari II malformations (also called Arnold-Chiari), in which parts of the cerebellum and brainstem are pulled downward into the foramen ovale owing to a spinal defect. The sonogram shows obliteration of the posterior fossa structures, usually with elongation of the cisterna magna (banana sign) and flattening of the frontal cephalic bones (lemon sign).

The cisterna magna is also located in the posterior fossa, posterior to the cerebellum. It is part of the openings of the subarachnoid space. Dilatation of the cisterna magna may occur in conditions such as Dandy-Walker malformation, in which there is an abnormal communication between the fourth ventricle and the cisterna magna via a defect in the cerebellar vermis.

The human brain has 4 choroid plexus. They are built as a filter between the blood and the cerebrospinal fluid (CSF) and consist of many blood vessels separated from the ventricular space by choroid epithelial cells that, by passive and active transport, maintain the balance of the cerebrospinal fluid. They are located in the superior horn of the lateral ventricles. On ultrasound, they appear as a bright half-moon–shaped echo area in the ventricle. Choroid plexus cysts are found in 1 per 100 fetuses with trisomy 18; however, if the only abnormal finding is a choroid plexus cyst, the risk for trisomy 18 is 1 in 477.

The American College of Obstetricians and Gynecologists (ACOG) recommends considering amniocentesis if there are other “soft” markers for aneuploidy, if the blood markers for aneuploidy are abnormal, or if the mother will be aged 32 years at the time of term delivery. A choroid plexus that is floating in the lateral ventricle is called a floating or “drooping” choroid, which is caused by an enlarged lateral ventricle.

The ventricular system of the brain is composed of the lateral ventricles and the third and the fourth ventricles. Cerebrospinal fluid (CSF) is produced by blood filtered by ependymal cells on the choroid plexus. CSF travels along all ventricles to be reabsorbed through the arachnoid villi.

During second-trimester ultrasound, the lateral ventricles are measured in a plane where the cavum septum pellucidum is visible anteriorly, from the internal membranes of the lateral ventricles just behind the choroid plexus. A blockage from the reabsorption system may cause an enlarged lateral ventricle, with increased CSF pressure, which is called ventriculomegaly, a condition seen with hydrocephalus. If the cause of the hydrocephalus is not evident, it is called simple hydrocephalus. In some cases, hydrocephalus may occur along with other conditions such as Dandy-Walker malformation or agenesis of the corpus callosum.

The cavum septum pellucidum is composed of two different layers of gray and white matter. The space between the two layers is the cavum, which disappears during adult life. It is closely related to the corpus callosum. Absence of the cavum on a second-trimester ultrasound may indicate absence of the corpus callosum, which may be associated with multiple syndromes, including trisomy 13 or 18 or Dandy-Walker malformation. Agenesis of the corpus callosum may be confirmed with ultrasound of the midcoronal planes.

Fetal anatomy: Spine

The spinal evaluation of the fetus begins with an anatomic assessment of the head, abnormalities of which reflect some spinal problems. For instance, Chiari II malformations are often seen in fetuses with spina bifida. Owing to the decreased pressure in the spine, contents of the posterior fossa herniate through the foramen magnum into the spinal canal. There is displacement of the cerebellar vermis, fourth ventricle, and medulla oblongata. The cistern magna enlarges, becoming banana-shaped. The frontal bones collapse and the head may take on a lemon shape.

The most common spinal anomalies are the ones associated with spina bifida, in which failure of the vertebral bones to close allows part of the spinal cord to lie outside the neural canal. In cases of spina bifida occulta, the vertebral schisis is covered with skin, and it may appear as a lipoma or dimple at birth. If the defect is open, it is referred as a spina bifida aperta, and it is further classified as a meningocele (if it is covered by a thin meningeal membrane) or myelomeningocele (if the sac contains neural tissue).

The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that, when possible, a longitudinal section of the fetal spine should be obtained to screen for open and closed spinal dysraphism. Targeted fetal neurosonography is recommended if there is suspicion of a brain or spinal abnormality during the obstetric ultrasound examination.[17]

Fetal anatomy: Chest

The fetal chest evaluation includes the fetal heart and lungs. The chest view should include a 4-chamber heart and outflow tracts, if possible. A detailed cardiac ultrasound is beyond the scope of this article and is reserved for patients with a prior history of a fetus with cardiac anomalies, patients with overt diabetes, suspicions of a syndrome that affects the fetal heart, and patients with abnormally appearing fetal cardiac anatomy based on second-trimester ultrasound.

In the evaluation of the fetal heart, the position of the fetal heart in relation to the fetal body should be assessed first. The fetal stomach and the apex of the fetal heart should be on the left side of the fetus.

Next, a 4-chamber view of the fetal heart should be obtained. The transverse view of the fetal heart is obtained right above the fetal diaphragm. With minor adjustments, a 4-chamber view is acquired. The apex should be pointing to the left side, and the right ventricle should be the most anterior chamber. The upper chambers should be equal in size to the lower chambers, and both sides should be of similar size during both systole and diastole. This view allows the sonographer to evaluate the interventricular septum. The size of the heart should be about half the size of the whole thorax.[18]

Movement evaluation during this view includes free movement of both atrioventricular valves, as well as the foramen ovale flap in the left atrium. In some cases, an echogenic intracardiac focus that represents calcifications on the papillary muscle is seen and is a marker for fetal aneuploidy. With this view, more than half of the congenital cardiac anomalies may be detected, including those related to cardiac position, cardiac septa, cardiac chambers, and masses. Left outflow and right outflow views may be obtained, if possible.

Additional cardiac views may be obtained for a more complete fetal cardiac evaluation. These include the long axis of the left ventricle, short axis of the great vessels, aortic arch, and pulmonary artery view.

Fetal heart tones may be visible as early as 5 weeks’ gestation. The 5 components of the fetal heart motion to be evaluated include rate, rhythm, atrioventricular association, structural anomalies, and evaluation for hydrops. The fetal heart rate may change during the course of the pregnancy but should generally be between 110 and 180 beats per minute. Routinely, an M-mode picture of the fetal heart is taken. Fetal heart rhythm may be constant or variable; minor variability is normal, but significant rate variability should be noted, and, if the fetus is viable, the obstetrician should be notified immediately.

Fetal heartbeat at 20 weeks' gestation. Fetal heartbeat at 20 weeks' gestation.

The fetal diaphragm is evaluated for continuity. Congenital diaphragmatic hernia is the most common anomaly in the fetal thorax if cardiac problems are excluded. They are difficult to observe with ultrasound tend to grow as the pregnancy progresses. The diagnosis may be made upon observation of loops of bowel in the thorax or discontinuity of the diaphragm muscle.

Cysts in the region of the lungs may result from cystic adenomatous hyperplasia or bronchopulmonary sequestration. There is hamartomatous involvement in cystic adenomatous hyperplasia, and the prognosis depends on the size and type. In bronchopulmonary sequestration, a portion of the bronchopulmonary system develops separately and is seen as a hyperechoic area at the lung base.

Fetal anatomy: Abdomen

Components of the abdominal-area evaluation should include presence, size, and position and of the stomach, kidneys, bladder, umbilical cord insertion site into the fetal abdomen, and umbilical cord vessels.

The plane for abdominal circumference should include the outer margins of a transverse plane that includes the fetal stomach, spine, liver, and the umbilical portion of the left portal vein equidistant from both sides of the abdomen. A small circumference may indicate a growth-restricted fetus. The fetal liver can be evaluated for size and presence of masses or cysts. Liver calcifications have been described as part of fetal viral infections. A “double-bubble” stomach indicates duodenal atresia, a marker for Down syndrome. Absence of the fetal stomach may be related to other fetal anomalies.

Esophageal atresia may be diagnosed with ultrasound based on the absence of a visible fetal stomach and the presence of polyhydramnios; however, the diagnostic yield with ultrasound is only about 50%. Bowel dilatation may indicate large-bowel obstruction or anal atresia. Hyperechoic bowel is seen in cases of meconium, bleeding, or cystic fibrosis.

Two anterior abdominal defects are worth mentioning. Omphalocele is a midline defect covered by a membrane. If the defect is large enough, it may include the fetal liver. The association with fetal syndromes is high. Gastroschisis is a lateral defect to the umbilical cord, not covered by a membrane. On ultrasound, it is visualized as free loops of bowel in the amniotic sac. It has been associated with some fetal syndromes, but at a lower rate than omphalocele.

Fetal kidneys are located on both sides of the fetal spine, with an ovoid shape in a longitudinal view and a round shape in a transverse view. Fetal kidneys enlarge with gestational age; therefore, they are easier to visualize as pregnancy progresses and the fetus grows. It is not routinely necessary to measure the kidneys. Care should be taken in not confuse the fetal adrenal gland as part of the kidneys, as it may be larger than in adults. Minor fluid collection in the kidneys is normal, but more than 4 mm before 32 weeks’ gestation or more than 7 mm after 32 weeks’ gestation fluid collection or pyelectasis may be a marker of aneuploidy.

In fetuses who present with obstruction of the renal outlet, the affected kidney is visualized on ultrasound as a paraspinal sac, cysts, or abnormal ureteral dilatations. Absence of kidneys is rarer but carries a lethal prognosis. Polycystic kidneys are a bilateral disorder representing tubular ectasia, usually appearing after the second trimester. Meckel-Gruber syndrome is characterized by polycystic kidneys with encephalocele and polydactyly.

Fetal bladder is seen as a hypoechoic area caudally on the fetus. The filling cycle of the bladder is 60-90 minutes; therefore if an initial view of the fetus does not demonstrate a bladder, repeat imaging is recommended. In rare cases such as in urinary obstruction, the identification of fetal sex is medically indicated.

The umbilical cord may be evaluated by applying color Doppler interrogation to the area around the bladder and visualizing the two umbilical arteries around the bladder. Care should be taken to obtain a transverse view of the fetal abdomen and not to confuse the fetal iliac arteries.

Fetal anatomy: Extremities

The presence or absence of fetal extremities should also be documented. This is done by demonstrating one bone in the proximal extremity and two bones distally. Measurement of the long bones is obtained to evaluate fetal growth.

A single-center study by Dicke et al indicated that although ultrasound examination allows most cases of abnormal hand position, limb reduction defect, and arthrogryposis to be detected prenatally, at least 20-25% of these abnormalities will probably be missed. The sensitivity of ultrasound was found to be as follows[19] :

  • Polydactyly: 19.1%

  • Abnormal hand position: 76.0%

  • Limb reduction defect involving the long bones: 76.1%

  • Arthrogryposis: 81.3%