Obstetric Ultrasound: Imaging, Dating, Growth, and Anomaly

Proper Gain and Focus

“Gain” refers to the lightness or darkness of ultrasound images. Acceptable images demonstrate the full range of grays that the machine is capable of producing. Gain is controlled by simply adjusting a knob found on all ultrasound machines. With improper gain, important detail is lost ( eFig. 9.1 ). Electronic control of the timing and order that crystals are activated can work to focus this beam at the region of interest within tissues. Image resolution is optimal when the structure of interest lies within this zone of optimal focus, which can be adjusted by the sonographer.

Optimal Magnification

Proper depth of the scanned field and magnification of areas of interest are important for several reasons. Limiting the size of the scanned area allows a higher frame rate and resolution. In addition, homing in on the area of interest draws attention to important detail within that scanned area ( eFigs. 9.2 and 9.3 ).

Attenuation and Shadowing

Propagation of ultrasound waves is affected by the medium through which the sound waves pass. For several reasons, ultrasound waves lose intensity as they pass through tissues. The pressure waves gradually diverge from the central beam and are scattered by reflection from small structures within the tissue, and part of the sound energy is absorbed within tissues. Some tissues, such as bone, strongly attenuate sound waves ( eFig. 9.4 ). The thicker the tissues through which sound waves must pass before arriving at the target, the more the attenuation and the greater the difficulty in retrieving good information from the echoes. Because of attenuation, obstetric ultrasound imaging is greatly affected by patient obesity. The use of “windows” in the maternal abdominal wall around the umbilicus and below the pannus can greatly enhance images ( eFig. 9.5 ). In early pregnancy or when structures of interest are low in the pelvis, the problem of attenuation is reduced considerably by the use of an endovaginal probe. When transvaginal ultrasound is used, a full bladder is usually not necessary.

Lower-frequency sound waves penetrate tissues better but cannot achieve the same resolution as higher-frequency oscillations. Available ultrasound probes operate over a range of frequencies (e.g., 1 to 5 MHz and 4 to 8 MHz for abdominal probes). The highest frequency probe that allows adequate penetration should be used. Because penetration of the maternal abdominal wall is not an issue with endovaginal ultrasound, probes used for this purpose usually operate at a frequency of 5 to 10 MHz.

Power Adjustment

Power, the energy that is emitted from the ultrasound, is different from gain, which is the degree of amplification of returning echoes. Increasing the power setting can improve penetration; however, this has much less effect on attenuation and optimal image production than does using the proper probe frequency or using natural windows. As will be noted below, increasing the power has possible implications for patient safety.

Proper Orientation

For sagittal views, the right of the screen corresponds to the inferior aspect of the patient ( Fig. 9.1 ). For transverse views, the patient's right is shown on the left of the screen. With transvaginal ultrasound, transverse views also show the patient's right side to the left of the screen. In transvaginal sagittal views, up is to the left of the screen (i.e., toward the bladder), and down (toward the sacrum) is to the right of the screen ( Fig. 9.2 ). Ultrasound transducers have a notch or ridge that demarcates the side of the probe that will correspond to the left side of the monitor. Thus in going from sagittal to transverse scans, the probe is always rotated counterclockwise, and it is rotated clockwise to move from transverse to sagittal.

A backward image is clearly unacceptable for diagnostics and for documentation. To establish the position of the fetus, the orientation of the probe must be correct. The sonographer should not depend on the side of the stomach or axis of the heart to define the position of the fetus because these structures are not always in a normal position. If the sonographer sits or stands on the wrong side of the patient or holds the probe backward, standard orientation of the images is difficult to maintain and it is difficult for the sonographer to develop the hand-eye coordination needed to quickly and accurately steer the probe.

Angle of Insonation

There is often a best angle from which to view aspects of the intrauterine contents and fetal structures. For example, transverse views of the kidneys are best obtained when the fetus is scanned through its back or front. When the angle of insonation is from the side of the fetus, one kidney is shadowed by the spine (see eFig. 9.4 ). As will be noted below, standard measurements of fetal urinary tract dilation require that the angle of insonation is through the fetal front or back. Optimal views of the brain, heart, and spine also require that the angle of insonation be appropriate. Sometimes, significant pressure with the transducer is needed to get the probe into position for optimal visualization.

M-Mode

For most obstetric applications, the familiar two-dimensional (2D) grayscale real-time ultrasound is used. This is formally known as B-mode ultrasound. Another ultrasound modality that is available on most machines is referred to as M-mode ultrasound (motion mode). M-mode ultrasound shows changes along a single ultrasound beam over time. M-mode is useful for documenting the presence and rate of fetal cardiac contractions ( Fig. 9.3 , eFig. 9.6 ) and is also used for specialized echocardiography applications, such as defining cardiac arrhythmias ( eFig. 9.7 ).

Color and Pulse-Wave Doppler

Doppler ultrasound is used to demonstrate the presence, direction, and velocity of blood flow. The machine displays moving blood as color superimposed on the 2D grayscale image. By convention, flow toward the ultrasound transducer is displayed in red and flow away is displayed in blue ( Fig. 9.4 ). Pulse-wave Doppler continuously measures the relative velocity of flow within a designated gate inside a vessel. Flow velocity waveforms are used to calculate the systolic/diastolic (S/D) ratio, the pulsatility index, and the resistance index. These indexes are primarily used to assess downstream resistance in the vessel being interrogated. In pregnancies with fetal growth restriction, the flow within the umbilical artery is used to assess placental function ( Fig. 9.5 ). For some applications, the absolute flow velocity is needed. For example, when screening for fetal anemia, the peak flow velocity in the fetal middle cerebral artery is measured, as this correlates with the degree of fetal anemia. To give meaningful results, the angle of insonation (θ) should be in line with the direction of blood flow ( eFig. 9.8 ).

Three-Dimensional Ultrasound

High-performance computers have allowed the development of ultrasound machines and probes that can acquire and process a three-dimensional (3D) volume. To obtain this volume, the transducer uses an internal mechanical sweep mechanism that summates contiguous 2D planes. This volume data can be stored for analysis of any desired plane through the scanned area. Information from an acquired volume may be processed in such a way that the fetal surface is displayed in a lifelike manner. Surface abnormalities, such as facial clefts ( eFig. 9.9 ), can be well demonstrated with this approach. 3D images can sometimes be more readily understood by patients and other professionals who will participate in care of the baby. Display settings can be adjusted to display the fetal skeleton, which can help visualize conditions such as open neural tube defects or skeletal dysplasias ( eFig. 9.10 ).

Despite the demonstrated capabilities of 3D ultrasound, no proof exists of an advantage of this technology over standard 2D imaging for prenatal diagnosis. A 2016 American College of Obstetricians and Gynecologists (ACOG) practice bulletin states that “three-dimensional ultrasonography may be helpful in diagnosis as an adjunct to, but not a replacement for, two-dimensional ultrasonography.”

In many communities, 3D ultrasound is available to patients in a nonmedical setting for entertainment or to provide keepsake images of the fetus. The use of ultrasound for nondiagnostic purposes has been condemned by the American Institute in Ultrasound in Medicine (AIUM), ACOG, and the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG). Concerns raised in their policy statements include possible adverse bioeffects of ultrasound energy, the possibility that an examination could give false reassurance to women, and the fact that abnormalities may be detected in settings where personnel are not prepared to discuss and provide follow-up for concerning findings.

First-Trimester Normal Findings

Both ACOG and AIUM have defined the essential components of a first-trimester scan. Knowledge of the time at which embryonic structures normally appear is important for identifying pathologic pregnancies. For the reasons noted earlier, it will be assumed that transvaginal ultrasound is being used for this discussion.

The gestational sac can usually be seen between 4 and 5 weeks, the yolk sac shortly thereafter, and the fetal pole with cardiac activity by 6 weeks ( eFig. 9.11 ). Cardiac activity can be seen simultaneously with the appearance of a fetal pole as a pulsation at the lateral aspect of the yolk sac. The fetal heart rate is initially quite slow, averaging 110 beats/min at 6 weeks, then it increases steadily to a mean peak of 157 beats/min at 8 weeks. Because pulse or color Doppler carries higher energy, which theoretically could be harmful during embryogenesis (before 10 menstrual weeks), M-mode ultrasound is used to document cardiac activity during this time ( eFig. 9.6 ). Starting at 7 weeks, the embryo has grown to the point that recognizable features, such as a cephalic pole, can be seen. As shown in eFig. 9.6 , a prominent midline brain vesicle can be seen at this time. The cerebral falx is visible at 9 weeks, and the appearance and disappearance of physiologic gut herniation are noted between 8 and 11 weeks ( eFig. 9.12 ). In the course of this physiologic process, the bowel is seen to lie within the umbilical cord and does not float freely. Obviously, the diagnosis of an abdominal wall defect should be made with caution at this age. The stomach can consistently be seen by 11 weeks. If conditions are favorable, it is often possible to also visualize the bladder and kidneys at 11 weeks. At about 12 weeks, using color Doppler, the two umbilical arteries can often be identified as they course around the bladder. When the fetal position is favorable, transvaginal ultrasound has the potential for giving good views of the fetal cardiac anatomy in most patients at 13 weeks. Until 13 to 16 weeks’ gestation, the amnion has not fused to the chorion and is seen as a separate membrane (see eFig. 9.12 ). Until 12 weeks, the crown-rump length (CRL) should be measured for gestational age determination. Care should be taken to measure the full length of the fetus. The gestational age can be significantly underestimated if an oblique plane is used ( eFig. 9.13 ).

First-Trimester Abnormal Findings

Spontaneous abortion occurs in 15% of clinically established pregnancies. When cardiac activity has been demonstrated, the miscarriage rate is reduced to 2% to 4% in asymptomatic low-risk women. It is important to note, however, that in some groups at high risk for miscarriage—such as women over the age of 35 years who are undergoing infertility treatments—early visualization of cardiac activity does not provide quite as much reassurance. In one study that involved such women, the miscarriage rate in asymptomatic women was still 16% after a heartbeat was documented. In younger women who present with bleeding, 5% to 15% miscarry if the ultrasound is normal and shows a live embryo. If an intrauterine clot is present coexistent with an otherwise normal-appearing pregnancy ( eFig. 9.14 ), the miscarriage rate is 15%.

In the majority of pregnancies destined to abort, the embryo does not develop, and ultrasound shows an empty gestational sac ( eFig. 9.15 ). Such a pregnancy is termed an anembryonic gestation . When a failed pregnancy is suspected based on clinical or sonographic grounds, patients and clinicians alike are anxious to determine viability as soon as possible. However, it is clearly unacceptable for a pregnancy to be incorrectly deemed non-viable, since a desired pregnancy might be interrupted. Because no significant medical risk attends waiting for certainty when a failed pregnancy is suspected, a cautious approach is always advisable. Criteria for deciding that an intrauterine pregnancy of uncertain viability is in fact a failed pregnancy were recommended by a multispecialty panel convened by the Society of Radiologists in Ultrasound. These include (1) the presence of a fetus with a CRL of more than 7 mm and no heartbeat, (2) the absence of an embryo when the mean sac diameter is greater than 25 mm, (3) the absence of an embryo with a heartbeat more than 2 weeks after a scan showed a gestational sac without a yolk sac, and (4) the absence of an embryo with a heartbeat more than 11 days after a gestational sac with a yolk sac was seen. These criteria have been endorsed by American and Canadian practice guidelines. Other findings that are suspicious, but not diagnostic, of pregnancy failure include a fetal heart rate of less than 85 beats/min at greater than 7 weeks' gestation, an enlarged yolk sac (>7 mm) ( eFig. 9.16 ), the presence of a mean sac diameter of 16 to 24 mm with no embryo, less than a 5 mm difference between gestational sac diameter and the CRL of the embryo, and slower development than expected but that do not meet the strict criteria described above. If there are borderline findings and uterine evacuation is being considered, it is prudent to repeat the ultrasound in 7 to 10 days to be absolutely sure that a viable pregnancy is not interrupted. Although a quantitative human chorionic gonadotropin (hCG) value that does not show an appropriate rise may indicate an abnormal pregnancy, a decision to medically or surgically evacuate a documented intrauterine pregnancy can most confidently be made by the ultrasound criteria described above.

First-trimester ultrasound findings predictive of a chromosome abnormality include a thick nuchal translucency, absent nasal bone, abnormally fast or slow fetal heart rate, and some structural malformations. The first-trimester aneuploidy screen is discussed in detail in Chapter 10 .

Types of Examinations

Major professional organizations that guide the use of ultrasound in obstetrics have defined a set of criteria for standard obstetric ultrasound examinations performed in the second and third trimesters. Components of a standard obstetric examination are shown in Box 9.1 . A complete description of guidelines can be found in the listed references.

These guidelines recognize that not all ultrasound examinations have the same purpose. For this reason, types of fetal sonographic evaluations have been defined. Components of the first-trimester ultrasound examination were described previously. A standard second- or third-trimester examination (current procedural terminology [CPT] code 76805), as defined in Box 9.1 , can be performed by any appropriately qualified sonographer. It is recognized that certain scans, termed specialized examinations (CPT code 76811), are more complex than the complete standard examinations performed in the course of routine pregnancy care. This designation and billing code are intended to be used for referral practices with special expertise in the identification of and counseling on fetal anomalies . Other specialized examinations may include fetal Doppler ultrasonography and fetal echocardiography. Follow-up examinations are needed for many obstetric conditions; these are termed repeat examinations (CPT 76816).

Another examination category is the limited examination (CPT 76815). Limited ultrasound examinations, also performed by an individual with appropriate training, are used to obtain a specific piece of information about the pregnancy. Examples include the determination of fetal cardiac activity and fetal lie, assessment of amniotic fluid volume, and measurement of cervical length. Unfortunately, it is all too common for practitioners to perform limited examinations in a manner inconsistent with good medical practice. For example, in some clinics, practitioners perform an ultrasound at the first prenatal visit to document viability but do not measure the fetus or record the results of the examination. Such a practice can create problems later in pregnancy when the gestational age is in doubt.

All the aspects of the standard obstetric examination listed in Box 9.1 are important for clinical management and should not be neglected. It is clearly unacceptable for an ultrasound exam to miss such conditions as placenta previa, multiple gestation, or an ovarian tumor. The diagnosis and management of these and other conditions are discussed in detail elsewhere in this book, but a brief description of the importance of the components of the standard ultrasound examination will be given here.

Qualifications for Performing and Interpreting Diagnostic Ultrasound Examinations

Most ultrasound exams in the United States are performed by professionals who are credentialed by the American Registry of Diagnostic Medical Sonography (ARDMS). Individuals with these credentials have had extensive education and testing to ensure their competency. A physician then generates a report based on the images and information obtained by the sonographer. When appropriate, the physician may personally perform or repeat parts of the exam. In a recent practice bulletin, ACOG sets forth criteria by which licensed medical practitioners are qualified to perform or interpret ultrasound examinations. They should be versed in the indications and expected content of a complete exam and have knowledge of the limitations of diagnostic ultrasound. They should understand the anatomy and pathophysiology of structures being evaluated and should have had specific training in obstetric sonography. Additionally, they are responsible for the quality, accuracy, documentation, and safety of exams performed under their supervision.

Cardiac Activity

Obviously, the presence or absence of cardiac activity should be documented and is usually the starting point of an exam. As noted previously, after about 6 weeks’ gestation, the diagnosis of fetal life is rarely difficult. Even though fetal death may be obvious with B-mode imaging, confirming the absence of a heartbeat with color or pulse-wave Doppler is recommended. Throughout pregnancy, an abnormally fast, slow, or irregular heartbeat can be detected by visual inspection using grayscale ultrasonography. The abnormal rate can be quantified and documented with M-mode (see Fig. 9.3 ) or pulse-wave Doppler ultrasound ( Fig. 9.6 ).

Number of Fetuses

When a multiple pregnancy is diagnosed, the number of amnions and chorions should always be determined (see Chapter 39 ). Determination of chorionicity is most easily accomplished in early pregnancy ( Fig. 9.7 ). The presence of unlike sex, separate placentas ( eFig. 9.17 ), or a thick membrane dividing the sacs with a twin peak or lambda sign ( Fig. 9.8 ) all indicate the presence of two chorions. With monochorionic twins there is a thin intervening membrane, which attaches to the placenta without a peak ( Fig. 9.9 ). The level of fetal risk is much higher when fetuses share chorions, and the risk is extremely high if there is a single amnion ( Fig. 9.10 , eFig. 9.18 ). Monochorionic pregnancies require early referral for a specialized ultrasound. Twin-to-twin transfusion syndrome occurs in 15% of these pregnancies and is associated with a high degree of morbidity and mortality. Because it may have an abrupt onset and is a treatable condition, most authorities recommend ultrasound every 2 weeks starting at 16 weeks. In all twin pregnancies, periodic ultrasound examinations should be performed to assess fetal growth. Twins are at significantly increased risk for growth abnormalities, and it is not possible to assess the growth of the twins individually by abdominal palpation.

Presentation

The assessment of presentation is not merely a matter of determining whether the fetus is head down or breech. A more precise ultrasound analysis of presentation is important in certain circumstances. A back down transverse lie makes cesarean delivery more difficult, and a back up transverse lie increases the risk of cord prolapse with premature rupture of membranes. The attitude of the fetal head, especially a face presentation, is important in assessing progress in labor. Ultrasound can be used to clarify the position of the fetal head in difficult cases with marked caput or molding.

Amniotic Fluid Volume

Every ultrasound examination should include an assessment of the amniotic fluid volume (see Chapter 28 ). It is acceptable for an experienced examiner to make this determination subjectively. However, to aid in communication and to provide criteria for management protocols, semiquantitative methods are preferred. The amniotic fluid index (AFI) is the sum of the measurements of the deepest vertical pocket (DVP) of fluid in each of the uterine quadrants ( Fig. 9.11 ). The limits of the quadrants are the maternal midline and a horizontal line through the maternal umbilicus. Each pocket should measure at least 1 cm in width. The line between the calipers should not cross through loops of cord or fetal parts. Polyhydramnios and oligohydramnios can be defined by either the AFI or measurement of the single DVP. Polyhydramnios is usually defined as an AFI greater than 24 cm or DVP of greater than 8 cm, and oligohydramnios as an AFI less than 5 cm or DVP less than 2 cm.

Oligohydramnios

The complete absence of amniotic fluid before labor can indicate fetal malformations, rupture of the membranes, or placental insufficiency ( eFigs. 9.19 and 9.20 ). A deficit of amniotic fluid that occurs before the mid second trimester can result in the oligohydramnios sequence , which includes pulmonary hypoplasia, fetal deformations, and flexion contractures of the extremities ( eFig. 9.21 ). The outcome with anhydramnios depends on the cause and the gestational age at which it is first present. Fetal malformations that cause absence of fluid usually involve the urinary tract. These defects will be described later in this chapter. Absence of urine excretion into the amniotic space from a structural abnormality leads inevitably to lethal pulmonary hypoplasia. When mid second trimester anhydramnios results from rupture of membranes or placental insufficiency, the development of pulmonary hypoplasia is less predictable.

Since oligohydramnios can be an important sign of placental insufficiency, assessment of amniotic fluid volume is part of the biophysical profile (see Chapter 27 ). Because of the association between oligohydramnios and fetal compromise, it became common practice to deliver the baby when the AFI was less than 5. However it has been shown that isolated ultrasound-diagnosed oligohydramnios is not as predictive of perinatal outcome as was previously thought. ACOG now recommends using a DVP of 2 cm or less as the definition of oligohydramnios by which clinical management decisions should be made . This method is simpler than the AFI, and more importantly, it has been shown to reduce the rate of obstetric interventions for oligohydramnios with no difference in perinatal outcomes compared with using the AFI. Because of its association with placental insufficiency oligohydramnios is usually considered an indication for antepartum testing, and may be an indication for early term delivery.

Polyhydramnios

Polyhydramnios has been classified as mild, moderate, or severe. Using the AFI, these categories are defined by measurements greater than 24 cm, 30 cm, and 35 cm, respectively. A single DVP of more than 8 cm, 12 cm, or 16 cm also can be used for categorization of polyhydramnios ( eFig. 9.22 ). Severe polyhydramnios may be indicative of a malformation or other fetal problem and requires specialized ultrasound. For many of these conditions, the excess amniotic fluid is a result of poor fetal swallowing—because of neurologic abnormalities, genetic syndromes, or gastrointestinal (GI) malformations. The chance of a malformation or genetic syndrome being present with mild, moderate, or severe polyhydramnios is approximately 8%, 12%, and 30%, respectively. The chance of a fetus with polyhydramnios having aneuploidy is 10% when other anomalies are present. Other serious causes of severe polyhydramnios include twin-to-twin transfusion syndrome and fetal hydrops. An association has been found between polyhydramnios and fetal macrosomia, and maternal diabetes mellitus is present in about 5% of cases. Mild polyhydramnios may simply be a variant of normal, and it often resolves spontaneously. With polyhydramnios, an increase in preterm birth is observed when the patient has diabetes (22%) or the fetus has anomalies (39%) but not when the polyhydramnios is idiopathic. Polyhydramnios has been linked to macrosomia and an increase in adverse pregnancy outcome, including stillbirth, and is considered an indication for antepartum testing. When polyhydramnios persists, follow-up ultrasound exams are appropriate to assess fetal growth and amniotic fluid volume.

Placenta and Umbilical Cord

One of the principal advantages of routine ultrasound is that serious problems of placentation—such as placenta previa, placenta accreta, and vasa previa—can be diagnosed in a timely manner (see Chapter 18 ). At the time of the routine screening ultrasound (beyond 18 weeks), it should be determined whether the placenta covers the internal cervical os ( Fig. 9.12 ). If the placenta and the cervix are not seen clearly ( eFig. 9.23 ), or if it appears that the edge of the placenta is close to the cervix, vaginal ultrasound should be used liberally to clarify this relationship ( Figs. 9.13 and 9.14 ).