Transvaginal Ultrasound

Transvaginal ultrasound is useful in the first trimester of pregnancy because the close proximity of the intravaginal ultrasonic transducer allows for high-frequency scanning and thus better resolution of the pelvic organs and developing pregnancy than transabdominal imaging. Transvaginal ultrasound is commonly used in the first trimester to determine accurate dating of the pregnancy as well as fetal location and number. The nuchal translucency measurement (first-trimester screening), a sonographically derived assessment of the subcutaneous fluid collection at the level of the fetal neck, is a screening test for chromosomal and structural abnormalities that is performed between 11 and 14 weeks’ gestation, typically by a transabdominal but also a transvaginal approach (see Figure 7-2, pg 81). First-trimester vaginal ultrasound can also identify structural malformations. Transvaginal sonographic measurement of cervical length in the mid-trimester can be used to identify patients at risk for preterm delivery. The median length of the cervix at 24 to 28 weeks is 3.5 cm. Patients with a cervical length less than 2.0 cm are at significantly increased risk for preterm birth (threefold to fivefold). Finally, transvaginal ultrasonic imaging of the lower uterine segment in the second or third trimester allows for very precise identification of placental location in relation to the internal cervical os. In a patient with vaginal bleeding, excluding placenta previa is important in management.

Transabdominal Ultrasound

After 16 weeks’ gestation, transabdominal ultrasound (second-trimester screening) is used to evaluate the fetus for structural abnormalities, provide a baseline assessment of fetal growth, and provide information regarding fetal well-being. The ability of a second-trimester scan to identify a fetus with an anomaly ranges from 17% to 74%. The reason various studies show such a wide range in sensitivity is probably due to variations in patient population and operator skill. The specificity, or the ability of ultrasound to correctly identify a normal fetus, approaches 100% in all studies. Thus ultrasound is useful in ruling out fetal anomalies, but it is not as reliable in detecting them.

In the third trimester, transabdominal ultrasound is useful in assessing fetal growth. Serial biometric measurements of the fetal head, abdomen, and limbs provide longitudinal information regarding the fetal growth trajectory. Software packages integral to the ultrasonic machines allow calculation of a fetal weight estimate from these measurements; this estimate is often used clinically. However, understanding that these estimates may have an error of ±15% (a variation of ±1 lb or 450 g in a 7-lb or 3400-g fetus) limits the utility of ultrasonic fetal weight, especially in larger (>8 lb or 4000 g) fetuses.

Ultrasonic visualization of aspects of fetal behavior (body movement, breathing) provides highly predictive information regarding fetal oxygenation and well-being. These aspects are combined to determine the biophysical profile (Box 17-2). The risk for fetal death within the week following a biophysical profile score of 8 or more is less than 1%.

Doppler Sonography

Doppler sonography, which can precisely measure the velocity profile of blood flowing through fetal vessels, allows for characterization of vascular impedance. The umbilical artery, which normally has high-velocity flow during cardiac diastole, may have low, absent, or even reversed diastolic flow in a compromised fetus with high-resistance placental vasculature. Similarly, because the peak flow velocity through a blood vessel is inversely proportional to the viscosity of the liquid flowing through it, Doppler studies of the fetal middle cerebral artery are used as a noninvasive estimate of fetal hematocrit. This is useful in management of severe fetal anemia in pregnancies complicated by isoimmunization.

Finally, ultrasound is used to assist in performing invasive obstetric procedures. Amniocentesis, chorionic villus sampling (CVS), and percutaneous umbilical blood sampling (cordocentesis) are examples of procedures that require continuous ultrasonic guidance.

Genetic Diagnosis

Amniocentesis for prenatal diagnosis of chromosomal anomalies is performed at 16 to 20 weeks of gestation. The procedure-related risks are an approximately 0.3% pregnancy loss rate and a 1% postprocedure measurable amniotic fluid leakage rate. Early amniocentesis done before the 15th week of gestation is associated with a higher miscarriage rate (3% to 4%), a higher postprocedure leakage rate (3%), and an additional risk for limb deformities, including clubfoot (1%). Amniotic cells require 1 to 2 weeks of culture before final chromosomal analysis is possible, although fluorescent in situ hybridization (FISH) can be used with chromosome-specific probes (e.g., trisomy 21, 18, and 13) and gives preliminary results in 3 days.

Single gene defects that have been characterized at the molecular level are amenable to prenatal diagnosis through amniocentesis. Using polymerase chain reaction (PCR), fetal DNA in the amniocytes can be amplified rapidly to allow for direct or indirect molecular analysis of genetic disorders. Examples of common prenatally diagnosed genetic disorders include cystic fibrosis, Tay-Sachs disease, sickle cell disease, and fragile X syndrome.

Biochemical Testing

An example of biochemical testing that can be performed on amniotic fluid is determination of the level of alpha-fetoprotein (AFP). AFP is a fetal serum protein that should, under normal circumstances, be detectable in the amniotic fluid in only trace amounts. In the event that the fetal dorsal or ventral wall is open (e.g., neural tube defect or gastroschisis), amniotic fluid AFP will be elevated, allowing detection of these defects even if ultrasonic imaging is equivocal or nondiagnostic.

Diagnosis of Perinatal Infections

In the United States the most commonly encountered prenatal infections with potential sequelae for the fetus include cytomegalovirus (CMV), parvovirus B19, varicella zoster virus (VZV), and toxoplasmosis