Umbilical Cord Abnormalities
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Direct answer
Umbilical cord abnormalities include a single umbilical artery, true knots, nuchal cord and velamentous insertion, and most are found incidentally on ultrasound or at delivery. A single umbilical artery matters most because it links with renal and cardiac anomalies and with growth restriction. Velamentous insertion becomes dangerous when fetal vessels cross the cervical os, creating a risk of vasa praevia and fetal haemorrhage when the membranes rupture.
What you must remember
- Normal cord: two arteries and one vein, so a two-vessel cord indicates a single umbilical artery.
- Single umbilical artery: seen in about 1 per cent of pregnancies, with renal, cardiac and growth associations.
- True knot: a genuine knot in the cord, more common in monoamniotic twins, which can compromise fetal flow when tightened.
- Nuchal cord: a loop around the neck, common at delivery and usually harmless unless it tightens during labour.
- Velamentous insertion: the vessels run unprotected in the membranes before reaching the placenta.
- Vasa praevia: unprotected fetal vessels crossing the internal os, which can tear when the membranes break.
- Cord length: a cord under about 35 cm is short and linked with abruption and reduced movements, while one over about 80 cm is long and risks prolapse and entanglement.
- Anomaly scan: the routine scan checks cord vessel number, and a single artery prompts a detailed anatomy and growth review.
Recognising and managing each cord variant
Detection route and clinical signal differ for each variant. The table compares how each is detected with what it signals for the pregnancy and delivery.
| Variant | Detection | Main implication |
|---|---|---|
| Single umbilical artery | Two-vessel cord on scan or at delivery | Detailed anomaly scan, renal and cardiac review, and growth surveillance |
| True knot | Suspected on scan, confirmed at delivery | Fetal monitoring in labour; tight knots can compromise fetal flow |
| Nuchal cord | Seen on scan or at delivery | Usually harmless; the loop is gently reduced over the head at birth |
| Velamentous insertion | Colour Doppler shows cord inserted into membranes | Vasa praevia risk if vessels cross the os; check before labour |
| Short or long cord | Measured on scan or at birth, with cord length recorded in the notes | Short: reduced movement and abruption; long: prolapse and entanglement |
Most variants are incidental, so the decision rests on the associated risk rather than on the finding alone. Vasa praevia is the one that changes management before labour, which is why transvaginal Doppler is used when velamentous insertion is suspected, and the finding is written into the delivery plan for the whole team.
Traps in cord questions
Cord questions usually test whether you recognise a harmless variant and a dangerous one.
- Trap: Treating a single umbilical artery as harmless Correct: it is linked with renal and cardiac anomalies, so a detailed anatomy scan and growth follow-up are needed.
- Trap: Calling a nuchal cord an indication for caesarean Correct: loose loops are common and usually reducible, and intervention depends on fetal heart rate changes.
- Trap: Missing vasa praevia in velamentous insertion Correct: vessels crossing the os can rupture when membranes break, so transvaginal colour Doppler is used before rupture.
- Trap: Confusing velamentous insertion with battledore placenta Correct: velamentous has vessels running in the membranes, while battledore has a marginal cord insertion on the placenta.
Frequently asked questions
What is the clinical risk of a single umbilical artery?
A single umbilical artery is associated with renal and cardiac anomalies, chromosomal abnormalities and fetal growth restriction. Affected pregnancies need a detailed anomaly scan and serial growth scans to pick up these associations.
Is a nuchal cord an indication for caesarean section?
A nuchal cord is not an indication for caesarean section on its own, because loose loops are common and usually harmless. Intervention depends on the fetal heart rate trace during labour, and a tight loop causing repeated decelerations may need prompt management.
What is vasa praevia and how does velamentous insertion lead to it?
Vasa praevia is fetal vessels crossing the internal os, unprotected by cord or placenta. Velamentous insertion of the cord is its commonest setting, and when the membranes rupture these vessels can tear and cause fetal haemorrhage.
How is a true knot of the umbilical cord identified?
A true knot is usually suspected on ultrasound when the cord shows a tight loop configuration, and it is confirmed after birth. It is most common in monoamniotic twins, and tight knots can cause fetal compromise, so reduced movements or abnormal monitoring need prompt review.