The umbilical cord contains the blood vessels that carry blood between the fetus and the placenta. It is encased by a membrane that is continuous with the amnion. Within this membrane lie the umbilical blood vessels bathed in Wharton’s jelly. The normal cord contains one vein and two arteries and is typically coiled, most often with a left spiral (the opposite of the right spiral of a screw). The jelly and coiling protect the cord from compression and kinking.

The umbilical arteries connect to the internal iliac arteries in the fetal pelvis. With each fetal systolic contraction, blood flows in the umbilical arteries away from the fetus toward the placenta. Blood returns to the fetus through the umbilical vein, which drains via the left portal vein into the inferior vena cava through the ductus venosus.

Except in cases of severe oligohydramnios, the umbilical cord is visible as a serpiginous structure surrounded by amniotic fluid, extending from the placenta (Figure 3.1.1) to the fetus (Figure 3.1.2). The cord can be seen to be coiled (Figure 3.1.3) or, less frequently,
straight (Figure 3.1.4). Visualization of the cord may be enhanced using color Doppler (Figures 3.1.1 to 3.1.6). If a transverse section through a single loop of normal umbilical cord can be imaged, one large and two small circles are seen, the former representing the larger umbilical vein and the latter the two smaller umbilical arteries (Figure 3.1.5).

The intrafetal extensions of these vessels can be imaged via color Doppler. In particular, the existence of two umbilical arteries can be confirmed by demonstration of two umbilical arteries adjacent to the fetal bladder (Figure 3.1.6).

Spectral Doppler of the umbilical arteries demonstrates pulsatile flow (Figure 3.1.7). Flow normally continues in a forward direction throughout the fetal cardiac cycle. The systolic/diastolic ratio tends to decrease as pregnancy proceeds and is normally less than or equal to 4 at 26–30 weeks gestation, 3.5 at 30–34 weeks, and 3 from 34 weeks onward.