Parturition

Parturition is the formal term for childbirth: the process by which the uterus expels the fetus and placenta at the end of gestation. It marks the transition from pregnancy to the postpartum period and is orchestrated by hormonal changes in both the mother and the fetus rather than by any single trigger.

Hormonally, parturition depends on a shift in the balance between progesterone, which keeps the uterine muscle quiescent during pregnancy, and factors that promote contraction. As term approaches, rising estrogen activity increases the excitability of the myometrium and upregulates receptors for oxytocin, the posterior pituitary hormone that stimulates uterine contractions. Prostaglandins soften and dilate the cervix and further strengthen contractions. Labor itself is a classic positive feedback loop: contractions push the fetus against the cervix, cervical stretch signals more oxytocin release, and more oxytocin produces stronger contractions — a cycle that continues until delivery ends the stimulus.

Labor proceeds in three stages: first, cervical dilation and effacement (the longest stage); second, delivery of the fetus through the fully dilated cervix; and third, delivery of the placenta. Afterward, continued oxytocin release helps the uterus contract down, limiting bleeding, and supports milk ejection during lactation.

On the MCAT, parturition is tested within the reproductive system content of the biology section. Know the roles of oxytocin, estrogen, progesterone, and prostaglandins, the three stages of labor, and — most heavily tested — parturition as a textbook example of positive feedback in human physiology.