Late in pregnancy, estrogen stimulates the production of prolactin, the hormone that starts milk production in the breasts, and the receptors for it. It also sparks a striking increase in the number of oxytocin receptors in the uterus. As its derivation from the Greek implies, oxytocin (“quick birth”) stimulates the smooth-muscle cells of the uterus to rhythmically contract, forcing out the baby during childbirth. By the time a woman goes into labor, she can have three hundred times more oxytocin receptors on those cells than she did before she got pregnant. Oxytocin is also needed to eject
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