The Umbilical Cord

Embryologic Development

The umbilical cord forms during the first 5 weeks of gestation (7 menstrual weeks) as a fusion of the omphalomesenteric (yolk stalk) and allantoic ducts ( Fig. 57.1 ). An outpouching from the urinary bladder forms the urachus, which projects into the connecting stalk to form the allantois. The allantoic vessels become the definitive umbilical vessels. The umbilical cord acquires its epithelial lining as a result of enlargement of the amniotic cavity and envelopment of the cord by the amniotic membrane. The intestines grow at a faster rate than the abdomen; they herniate into the proximal umbilical cord at approximately 7 weeks and remain there until approximately 10 weeks. The insertion of the umbilical cord into the ventral abdominal wall is an important sonographic anatomic landmark because scrutiny of this area will reveal abdominal wall defects, such as omphalocele, gastroschisis, or limb–body wall complex.

Normal Anatomy

The umbilical cord is covered by the amniotic membrane. The cord includes two umbilical arteries and one umbilical vein ( Fig. 57.2 ) and is surrounded by a homogeneous substance called Wharton jelly. Wharton jelly is a myxomatous connective tissue that varies in size and may be imaged with high-frequency ultrasound transducers. The diameter of the cord usually measures 1 to 2 cm (variations in cord diameter are usually attributed to Wharton jelly). The normal length of the cord is 40 to 60 cm; it is difficult to assess the length reliably with ultrasound.

The umbilical arteries arise from the fetal internal iliac arteries, course alongside the fetal bladder, and exit the umbilicus to form part of the umbilical cord. The paired umbilical arteries course along the entire length of the cord in a helicoidal fashion surrounding the umbilical vein. The umbilical arteries branch along the chorionic plate of the placenta.

The umbilical vein is formed by the confluence of the chorionic veins of the placenta, with its primary purpose to transport oxygenated blood back to the fetus. The umbilical vein enters the umbilicus and joins the left portal vein as it courses through the liver. The intraabdominal portions of the umbilical vessels degenerate after birth; the umbilical arteries become the lateral ligaments of the bladder, and the umbilical vein becomes the round ligament of the liver.

Sonographic Evaluation of the Umbilical Cord

The umbilical cord has one large vein and two smaller arteries ( Fig. 57.3 ). The umbilical vein transports oxygenated blood from the placenta, and the paired umbilical arteries return deoxygenated blood from the fetus to the placenta for purification. The umbilical cord is identified at the cord insertion into the placenta and at the junction of the cord into the fetal umbilicus ( Fig. 57.4 ). The arteries spiral with the larger umbilical vein, which is surrounded by Wharton jelly. Absent cord twists may be associated with decreased fetal movement and a poor pregnancy outcome ( Fig. 57.5 ).

In the second and third trimesters, the two arteries and the vein can be clearly seen. The number of umbilical arteries can be distinctly identified and should be documented. The three vessels of the cord may be followed with real-time ultrasound as they enter the abdomen and travel toward the liver and iliac arteries ( Fig. 57.6 ). From the left portal vein, umbilical blood may flow through the ductus venosus to the systemic veins (inferior vena cava or hepatics), bypassing the liver, or through the right portal sinus to the right portal vein. The ductus venosus forms the conduit between the portal system and the systemic veins.

Sonographically, the ductus venosus appears as a thin intrahepatic channel with echogenic walls. ( Fig. 57.7 ) It lies in the groove between the left lobe and the caudate lobe. The ductus venosus is patent during fetal life until shortly after birth, when transformation of the ductus into the ligamentum venosum occurs in the second week after birth.

The umbilical arteries may be followed caudally from the cord insertion, in their normal path adjacent to the fetal bladder, to the iliac arteries. On sonography, the sonographer may look at the cord in a transverse plane to see one large umbilical vein and two smaller umbilical arteries. Another sonographic approach to viewing the arteries is to look lateral to the fetal bladder in a transverse or coronal plane. The umbilical arteries run along the lateral margin of the fetal bladder and are well imaged with color flow Doppler ( Fig. 57.8 ). In the postpartum stage, the umbilical arteries become the superior vesical arteries .

Omphalocele

Omphalocele occurs 1 in 5000 births and results from failure of the intestines to return to the abdomen. The omphalocele is categorized on whether the sac contains liver or only bowel. This hernia may consist of a single loop of bowel, or it may contain most of the intestines ( Fig. 57.12 ). The covering for the hernia sac consists of epithelium derived from the parietal peritoneum and amnion. The umbilical cord inserts into the base of the omphalocele sac.

There is a strong association of omphalocele with aneuploidy; more than half of the fetuses with omphaloceles have additional malformations and chromosomal abnormalities. Trisomy 18 is the most common associated aneuploidy, followed by trisomy 13.

Gastroschisis

Gastroschisis is usually a right paraumbilical defect involving all layers of the abdominal wall and usually measuring 2 to 4 cm. The small bowel always eviscerates through the defect ( Fig. 57.13 ). The loops of bowel are never covered by a membrane; thus they are directly exposed to amniotic fluid and elevated alpha-fetoprotein levels. Other organs that may eviscerate are large bowel, stomach, a portion of the gastrointestinal system, and, rarely, liver.