Which factor is listed as a contributor to the onset of labor?

• A. Decrease in estrogen
• B. Prostaglandin production in the myometrium
• C. Progesterone withdrawal
• D. Release of oxytocin by the pituitary

Answer: (B)

Prostaglandin production in the myometrium acts as a key trigger for labor because these lipid compounds directly increase uterine contractility and cervix readiness. Prostaglandins, especially PGE2 and PGF2α, raise the myometrium’s excitability by boosting calcium availability, promote the formation of gap junctions that coordinate contractions, and upregulate oxytocin receptors. They also drive cervical remodeling, softening and dilation that are essential for labor to progress. While estrogen supports labor by enhancing these effects and oxytocin released later helps sustain contractions, the direct initiating role lies with prostaglandins in the uterus.