Forward shift in the initiation of the nocturnal estradiol surge in the pregnant baboon: is this the genesis of labor?

Abstract

Daily (9 AM and 6 PM) blood samples were obtained from the inferior vena cava during the last trimester of pregnancy in a tethered baboon model. In addition, three 24-hour (hourly blood sampling) studies were performed at days 143 to 147, 158 to 162, and 172 to 177 of pregnancy. Dramatic 24-hour rhythms in progesterone and estradiol were detected, with both steroids surging nocturnally. Early in the third trimester the estradiol surge followed the progesterone surge. However, approximately 10 to 12 days before delivery, the initiation of the nocturnal estradiol surge shifted forward, thus preceding the progesterone surge. This forward shift in the estradiol surge created a daily (3 to 5 hours) window of elevated estradiol-to-progesterone ratio and appears to coincide with the initiation of nocturnal uterine contractions. The nocturnal uterine contractions can be inhibited by an oxytocin antagonist. We hypothesize that this forward shift in the initiation of the estradiol surge induces nocturnal uterine contractions by oxytocin release and/or increase in uterine oxytocin receptors and generates molecular messages that are the genesis for labor and delivery in the baboon.