Final answer:
The hormone that stimulates uterine contractions during childbirth is oxytocin. It is released from the posterior pituitary gland and stimulates increasingly stronger contractions through a positive feedback loop, essential for the progression of labor and the milk ejection reflex.
Step-by-step explanation:
The hormone responsible for stimulating uterine contractions during childbirth is oxytocin. Produced by the hypothalamus and released by the posterior pituitary gland, oxytocin plays a critical role in initiating labor. At the end of pregnancy, the uterus becomes more sensitive to oxytocin due to the increase in oxytocin receptors. Once labor begins, oxytocin stimulates the smooth muscle cells in the uterus to contract more intensely. This leads to the baby pressing against the cervix, which in turn signals for even more oxytocin to be released, creating a positive feedback loop that continues until the birth is complete. In addition to labor contractions, oxytocin is also vital for the milk ejection reflex during breastfeeding.
Estrogen also plays a role by stimulating the production of oxytocin, while progesterone inhibits its production. As labor approaches, the contractions become stronger due to the combined effects of oxytocin and prostaglandins, which the placenta releases into the uterus. If necessary, a synthetic form of oxytocin, known as pitocin, may be administered to induce labor.