Declining plasma progesterone concentration with advancing gestation in blood from umbilical uterine veins and fetal heart in monkeys

Abstract

No fall in maternal progesterone values preceding the onset of labor has been demonstrated in pregnant monkeys in plasma from a peripheral vein, giving rise to the question of how this hormone maintains the uterus in a quiescent state during gestation without blocking parturition. We wondered whether progesterone levels taken from sites more directly related to the uterus would yield different results. Plasma progesterone values were determined during the last half of gestation in pregnant monkeys in samples taken from the uterine veins, the umbilical vein, the fetal heart and a maternal peripheral vein. Peripheral vein levels during this period ranged from 1.3 to 18.0 ng/ml and showed no trend with advancing gestation. Levels in the uterine vein ranged from 2.2 to 150 ng/ml, in the umbilical vein from 6.9 to 233 ng/ml, and in the fetal heart from 3.8 to 223 ng/ml. In plasma from these 3 sites there was a significant trend toward lower values with advancing gestation. Uterine and umbilical vein progesterone levels appear more likely to reflect conditions in the uterus than maternal peripheral vein levels. Their declining values in the latter part of gestation suggest that, contrary to the indications afforded by peripheral levels, the pregnant macaque does not differ from the pregnant rabbit, cow, sheep and rat in experiencing a waning influence of progesterone on the uterus as term approaches.