Step 1: Understanding the Concept:
The process of childbirth (parturition) is governed by a neuroendocrine reflex mechanism. When the fetus is fully developed, it coordinates with the placenta to issue mild uterine contractions called the fetal ejection reflex. This reflex triggers the maternal pituitary gland to release specific hormones that establish a powerful, self-reinforcing positive feedback loop to push the baby out.
Step 2: Detailed Explanation:
Let's analyze the biological actions of the hormones listed in the options:
- Oxytocin (Option A): Secreted by the posterior pituitary gland, this hormone acts directly on the smooth muscle layer of the uterus (myometrium), causing strong contractions. These contractions stimulate further secretion of oxytocin, creating a positive feedback loop that intensifies labor. If a patient experiences weak contractions, doctors inject synthetic oxytocin (pitocin) to stimulate and induce safe labor.
- Vasopressin (Option B): Also known as Antidiuretic Hormone (ADH), its main job is to help the kidneys retain water and regulate blood pressure, not to contract the uterus.
- Relaxin (Option C): Relaxes the pelvic ligaments and softens the cervix to allow the birth canal to widen, but it does not drive active muscular contractions.
- Prolactin (Option D): Stimulates the mammary glands to produce milk after birth, playing no role in uterine wall contractions.
Thus, oxytocin is the correct clinical hormone choice to induce delivery.
Step 3: Final Answer:
The doctor could inject Oxytocin, matching option (A).