Umbilical cord growth in human and rat fetuses: evidence against the "stretch hypothesis".

A total of 103 human fetuses between the 7th and 30th week of gestation were obtained from induced abortion (40 fetuses were normal and 63 were abnormal), and the umbilical cord length (UCL) was measured. The UCL increased almost linearly with gestational age among normal fetuses, contrary to the commonly held tenet that the UCL increases exponentially during the second trimester. When UCLs from the 63 abnormal fetuses were compared with those of normal fetuses, 15 fetuses were found to have short UCL and 10 fetuses to have long UCL. Among the 15 fetuses with short UCL, 6 had early amnion rupture syndrome. An unexpected finding among the 10 with long UCL was that 8 of them had oligohydramnios. It has been suggested that the UCL increases in response to tensile forces placed upon it ("stretch hypothesis"); however, our results are inconsistent with this hypothesis because fetuses with oligohydramnios should be less active and their umbilical cords be subject to less stress. In a separate experiment, we studied the normal development of the UCL in rat fetuses and observed an almost linear increase during the whole gestation similar to that seen in humans. This finding is also inconsistent with the "stretch hypothesis" because amniotic fluid volume decreases significantly from day 19 of gestation to term in rats.