In late pregnancy, rising levels of which hormone increase the uterus's sensitivity to oxytocin and help start labor?

• A. Estrogen
• B. Progesterone
• C. Prostaglandins
• D. CRH

Answer: (A)

Estrogen. In late pregnancy, rising estrogen levels prepare the uterus for labor by increasing its responsiveness to oxytocin. It does this mainly by upregulating oxytocin receptor expression in the myometrium and promoting gap junction formation between smooth muscle cells, enabling coordinated, stronger contractions when oxytocin is present. Progesterone generally keeps the uterus relaxed, and its relative withdrawal at term allows estrogen to exert these effects. Prostaglandins aid cervical ripening and contractions, and CRH rises toward term to help time labor, but the specific boost in sensitivity to oxytocin comes from estrogen-driven receptor upregulation.