Doppler Ultrasound: Select Fetal and Maternal Applications

Number of Vessels

The number of umbilical arteries (UAs) should routinely be documented during ultrasound examination. There are several different techniques. One of the most common methods is to document two UAs traveling around the fetal bladder. To achieve this image, the umbilical cord insertion to the fetus is visualized in a cross section of the fetal abdomen. The color Doppler box is then placed over the fetal abdomen, and the ultrasound probe is angled toward the fetal pelvis while the cord insertion is kept in view. Two UAs are visualized with color Doppler traveling around the fetal bladder. UAs arise from the anterior branch of the internal iliac arteries; if the wrong angle is used, one or both internal iliac arteries may be visualized and can be mistaken for UAs ( Fig. 17.3 ). The alternative method is to identify a cross section of a freely floating loop of cord, but it is essential to be sure to obtain a true cross section to avoid artifact as a single artery loops around the umbilical vein.

Doppler Flow Pattern

To assess the flow pattern, a freely floating segment located approximately at the middle of the umbilical cord is identified. Attention should be paid to avoid compression of the umbilical cord between the extremities or against the uterine wall. Such external compression may affect the flow pattern by changing the vascular resistance.

Once a target loop is identified, the color Doppler box is placed over the umbilical cord with as close as possible to a 0-degree incident angle between the ultrasound beam and the vessel. The UAs can be visually identified based on their number, pulsatile color flow pattern, caliber, and direction of blood flow. The color flow scale and gain are adjusted appropriately to ensure that the velocity of the flow in question is within the scale. A pulsed Doppler gate is placed over the UA. The gate size is adjusted appropriately to sample only the UA (unless the intent is to sample both the UA and the umbilical vein). Most of the indices are calculated ratios (e.g., S/D, RI, PI) and do not require calculation of absolute flow velocities. However, the angle of insonation (i.e., the angle between the ultrasound beam and the direction of flow) should still be kept as low as possible during Doppler assessment to maximize the sizes of both systolic and diastolic flow components (see Fig. 17.1 and Video 17.1 ).

Fetal Growth Restriction

During normal pregnancy, there is a progressive increase in end-diastolic velocity in the UA due to decreasing downstream impedance to flow as the placental vessels grow. This causes all of the UA Doppler indices to become progressively lower across the third trimester ( Fig. 17.4 ).

There is a progressive worsening of the UA FVW due to increasing placental impedance in growth-restricted pregnancies, with risk of stillbirth and asphyxia. Use of UA Doppler studies in small-for-gestational-age pregnancies have been associated with reduction in perinatal mortality. However, these changes in Doppler waveforms should be seen as a marker of a high-risk condition and an indication for further surveillance and should not be used in isolation as an indication for delivery ( Fig. 17.5 ). Routine use of UA Doppler ultrasound in low-risk or unselected populations does not improve neonatal or maternal outcomes.

Monochorionic Twin Gestations

Monozygotic twins account for 3 to 5 of every 1000 deliveries. Two-thirds of monozygotic twins are monochorionic. The monochorionic placenta is functionally divided into regions that support each fetus individually. Up to 95% of monochorionic placentas have vascular connections between the placental regions that support each fetus. Disproportional placental sharing and unbalanced intertwin blood transfusion can lead to twin-twin transfusion syndrome (TTTS), selective fetal growth restriction (sFGR), or twin anemia-polycythemia sequence (TAPS) (see Chapter 37 ).