True knot of the umbilical cord.

Fifty umbilical cords were prepared and examined and their venous perfusion pressures measured with and without a true knot in the cord. Contrary to information in the literature, a loose umbilical cord knot did not affect the venous perfusion pressure. With a tightened knot, the smaller the umbilical cord diameter, the greater was the pressure required to perfuse past the knot. The umbilical vessels, protected by the myxomatous structure of the Wharton's jelly, were rarely completely occluded. These findings correlate clinically with the relatively high incidence yet low fetal mortality rate actually due to a true knot in the umbilical cord.