19.3 Umbilical Cord & Cervical Assessment

Normal Umbilical Cord Anatomy

The normal umbilical cord contains three vessels: two umbilical arteries and one umbilical vein, suspended in Wharton jelly. On cross-section the vein is the single large thin-walled vessel and the two arteries are smaller with thick muscular walls; the arteries carry deoxygenated blood away from the fetus, and the vein carries oxygenated blood toward it. Simply counting vessels in a free loop of cord can be misleading because the arteries may fuse near the placental insertion, so the standard confirmation is to image the fetal pelvis with color Doppler and demonstrate two umbilical arteries coursing around either side of the fetal bladder. The cord normally inserts near the center of the placenta; marginal (battledore) and velamentous insertions are abnormal variants covered in placental assessment.

Two-Vessel Cord (Single Umbilical Artery)

A single umbilical artery (SUA) occurs in roughly 0.5-1% of pregnancies. Sonographically only one artery and one vein are seen, and color Doppler at the bladder shows a vessel on only one side. An isolated SUA may be benign, but because of its associations it triggers a detailed anatomic survey and growth surveillance. Key associations include:

• Trisomy 18 (and other aneuploidies such as trisomy 13)
• Renal anomalies and cardiac anomalies
• IUGR and prematurity

Cord Accidents and Emergencies

• True knot — uncommon (well under 1% of deliveries); usually loose and asymptomatic, but tightening can obstruct flow; sonography may show a focal cord bunching but knots are frequently missed prenatally
• Nuchal cord — a loop around the fetal neck, present in roughly a quarter of term deliveries; a single loose loop is usually insignificant, but multiple tight loops are documented because they can affect heart rate in labor
• Cord prolapse — the cord descends below the presenting part, typically after membrane rupture with a high presenting part, malpresentation, or polyhydramnios. The presenting part then compresses the cord against the maternal pelvis, causing acute fetal hypoxia — an obstetric emergency requiring immediate delivery
• Vasa previa — fetal vessels from a velamentous cord insertion or a succenturiate lobe run unprotected through the membranes directly over the internal os. When membranes rupture, these vessels tear and the fetus can exsanguinate within minutes, with historically high fetal mortality when undiagnosed. Diagnosis is made by placing color Doppler over the internal os and demonstrating fixed fetal vessels crossing it, often confirmed with pulsed Doppler showing a fetal heart rate. It is managed with planned cesarean delivery before membrane rupture

Cord Masses

Cord cysts are either true cysts (remnants of the allantois or omphalomesenteric duct, with an epithelial lining) or pseudocysts (localized edema of Wharton jelly). Isolated first-trimester cysts often resolve, but persistent cysts are associated with trisomy 18. An umbilical vein varix is a focal dilatation of the vein, usually in the intra-abdominal segment, and warrants growth follow-up.

Cervical Assessment

Cervical length is measured transvaginally with an empty bladder: the probe is placed in the anterior fornix, pressure is minimized to avoid falsely lengthening the cervix, and the closed canal is measured from internal to external os. The shortest of three adequate measurements is used.

• Normal: cervical length greater than 25 mm before 24 weeks
• Short cervix: 25 mm or less, associated with increased risk of preterm birth
• Funneling — V- or U-shaped opening of the internal os with amniotic fluid or membranes protruding into the canal; the funnel is excluded from the length measurement

Cervical insufficiency (incompetence) is painless cervical dilation, classically presenting with a history of recurrent mid-trimester losses or procedures. Sonography shows a short cervix, funneling, or membranes bulging into the canal. Treatment is cerclage — a suture (McDonald or Shirodkar) placed around the cervix — after which sonography documents suture position and residual canal length above the stitch.

A few structural details round out cord assessment. The normal cord measures about 50-60 cm at term and normally shows a spiral twist; hypercoiling and absent coiling have both been linked to adverse outcomes including growth restriction. Marginal (battledore) insertion places the cord at the placental edge and is usually insignificant, whereas the velamentous variant — vessels splaying through membranes before reaching the disc — is the setup for vasa previa and vessel compression. When a nuchal cord is seen, color Doppler confirms the number of loops and whether flow is compromised; multiple loops should be documented in the report. In suspected cord prolapse, temporizing measures while awaiting cesarean include placing the patient in Trendelenburg or knee-chest position and manually elevating the presenting part off the cord. Cervical assessment has its own pitfalls: a full maternal bladder and excessive probe pressure both falsely lengthen the cervix, and a lower-segment contraction can mimic funneling — real-time observation over several minutes distinguishes a true dynamic change. Risk factors for cervical insufficiency beyond prior mid-trimester loss include cervical surgery (cone biopsy, loop electrosurgical excision procedure), in-utero diethylstilbestrol exposure, and uterine anomalies. Elective, history-indicated cerclage is typically placed at 12-14 weeks, whereas an ultrasound-indicated (rescue) cerclage follows documented shortening. After cerclage, sonographers measure the closed canal above the suture and the suture-to-external-os distance; a suture positioned near the internal os with at least 10 mm of closed canal above it is the desired result.