ETIOLOGY OF UMBILICAL VASCULAR COILING

A coiled umbilical cord and the surrounding Wharton's jelly results in a cord that is more resistant to torsion, compression and stretch when compared with a non-coiled cord. The helical smooth muscle fibers of the umbilical arteries is thought to provide an axis for coiling (1)

Theories proposed as a cause for vascular coiling include:

1. Fetal rotational movements (2) (active or passive).
2. Fetal hemodynamic forces (3). Normal coiling can be impeded by any factor that restricts normal fetal movement:
• Extrinsic factors i.e. intrauterine constraint (1,4).
• Intrinsic to the cord itself:
• Unequal growth of the umbilical arteries.
• Differential blood flow through the left and right umbilical arteries. The right umbilical artery is larger than the left one possible explaining why the direction of coiling is 7:1::Left:Right.

There is a significant lack of coiling in single umbilical artery suggesting that coiling is dependent on torque resulting from differential blood flow between the two umbilical arteries.

The normal helix is well established by 9 weeks gestational age (2).
The number of twists are thought to be constant throughout pregnancy (3).

REFERENCES

1. Strong TH, Finberg HJ, Mattox JH. Antepartum diagnosis of noncoiled umbilical cords. Am J Obstet Gynecol 1994;170:1729-1732.
2. Edmonds HW. The spiral twist of the normal umbilical cord in twins and singletons. Am J Obstet Gynecol 1959;67:102-120.
3. Malpas P, Symonds EM. Observations on the structure of the human umbilical cord. Surg Gynecol Obstet 1966;123:746-750.
4. Lacro RV, Jones KL, Benirschke K. The umbilical cord twist: Origin direction and relevance. Am J Obstet Gynecol 1987;157:833-838.