Nuchal cord and umbilical cord knots

Definition

• A nuchal cord is present when the umbilical cord is wrapped 360 degrees around the fetal neck; loose and tight as well as single and multiple loops are distinguished.
• A true cord knot is a genuine knotting of the umbilical cord; in monoamniotic twins the two cords can become entangled with each other.

Occurrence & epidemiology

Epidemiology

• Nuchal cords occur in about 10 to 29 per cent of fetuses and increase with gestational age, from about 6 per cent at 20 weeks to 29 per cent at 42 weeks; two or more loops affect 2.4 to 8.3 per cent of pregnancies.
• True knots occur in 0.04 to 3 per cent of all births.
• In monochorionic monoamniotic twins cord entanglement is very common, in one small study in all 18 pregnancies.

Aetiopathogenesis

Aetiology and risk factors

• The umbilical cord averages 50 to 60 cm in length; long cords (over 70 cm) are associated with more frequent entanglement, true knots and torsion.
• True knots usually form early in pregnancy (about 9–12 weeks), when amniotic fluid volume is large relative to the fetus, by the fetus somersaulting through a loop of cord, and can gradually tighten later.
• Predisposing factors are a long cord, polyhydramnios, excessive fetal movements, gestational diabetes, multiparity, male fetal sex and chronic hypertension.

Clinical features

Clinical features and complications

• Most nuchal cords are not associated with perinatal morbidity or mortality; problems arise mainly with a tight nuchal cord.
• A tight nuchal cord first compresses the thin-walled umbilical vein while blood continues to be pumped out of the infant through the thicker-walled arteries; the result is hypovolaemia, acidosis and anaemia.
• Loose knots do not affect venous perfusion pressure; as they tighten, resistance rises; true knots are associated with a 4- to 10-fold increased risk of stillbirth.

Diagnosis

• Colour and power Doppler and 3D sonography improve prenatal detection of true knots; because the whole cord cannot be visualised, many knots remain undetected until birth, and false-positive findings are possible.
• Cord entanglement in monoamniotic twins can be detected sonographically from about 10 weeks.

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Further reading (selection)

1. StatPearls: Embryology, Umbilical Cord (NCBI Bookshelf)
2. Matern Health Neonatol Perinatol 2017: Nuchal cord and its implications (PMC, open access)

Cross-references