Final answer:
Oxytocin is the hormone responsible for the contractions during labor and the expulsion of the fetus at birth.
Step-by-step explanation:
The hormone responsible for the contractions during labor and the expulsion of the fetus at birth is oxytocin.
Toward the end of the third trimester, the hormone estrogen causes receptors on the uterine wall to develop and bind to oxytocin. When the baby reorients and engages the cervix, nerve impulses are sent to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. Oxytocin causes the smooth muscle in the uterine wall to contract, while the placenta releases prostaglandins into the uterus, further increasing the contractions.
A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to ensure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force, eventually leading to the expulsion of the fetus.