Final answer:
Estrogen upregulates gap junctions in the myometrium, enabling synchronized contractions during labour, which are intensified by a feedback loop involving oxytocin and prostaglandins.
Step-by-step explanation:
Immediately before delivery, estrogen stimulates the upregulation of gap junctions between individual myometrial smooth muscle cells, which heightens their excitability and communication/coordination for contractions. As the pregnancy advances into the third trimester, progesterone levels plateau and decrease, leading to a relative increase in estrogen. This shift in hormone balance sensitizes the myometrium to contractile stimuli. Additionally, increased fetal cortisol enhances placental estrogen secretion, which further promotes the formation and function of gap junctions. These gap junctions are essential for the synchronization of myometrial cell contractions necessary for efficient labour and delivery. Labour and birth, driven by uterine contractions, are enhanced by the hormone oxytocin, which is released from the posterior pituitary. Estrogen primes the uterus by upregulating oxytocin receptors, while oxytocin and prostaglandins, produced by the placenta, work in a positive feedback loop to intensify contractions. This seamless coordination is vital for the successful expulsion of the fetus and placenta, marking the culmination of childbirth.