Which hormone's release by the pituitary is associated with the onset of labor?

• A. Oxytocin
• B. Progesterone
• C. Estrogen
• D. Prolactin

Answer: (A)

Oxytocin, released from the posterior pituitary, is the hormone most closely linked to the onset of labor. It is produced in the hypothalamus and released in response to signals from the cervix and uterus as labor begins. When cervical dilation and uterine stretch occur, a neuroendocrine reflex triggers more oxytocin release. Oxytocin then binds to receptors on the uterine smooth muscle, promoting rhythmic, strong contractions. As pregnancy advances, estrogen increases the density of these oxytocin receptors and enhances uterine responsiveness, amplifying contractions and prostaglandin production. This creates a positive feedback loop: stronger contractions cause more cervical dilation, which stimulates even more oxytocin release until birth. After delivery, oxytocin also facilitates milk ejection, but its key role in labor is driving the contractions that lead to delivery.