What hormone is primarily responsible for initiating labor?

The correct answer highlights the essential role of hormones in initiating labor, particularly the rise in estrogen levels coupled with a decrease in progesterone levels. As pregnancy progresses, estrogen levels increase significantly, leading to various physiological changes that prepare the body for labor. This increase in estrogen has several key effects, including enhancing the sensitivity of the uterus to oxytocin, which is crucial for uterine contractions.

At the same time, a decrease in progesterone, which is known for its role in maintaining pregnancy, creates a hormonal environment conducive to the onset of labor. Progesterone typically inhibits uterine contractions, and its reduction allows the uterus to become more responsive to contractions, signaling the body that it is time to initiate labor.

In this context, while oxytocin is indeed important for stimulating contractions during labor, it is primarily influenced by the other hormonal changes noted in the correct answer. Relaxin also plays a role in preparing the cervix and pelvis for birth but is not the primary initiator of labor. Prolactin, meanwhile, is mainly involved in milk production and does not have a direct role in the initiation of labor. Thus, the combined effect of increased estrogen and decreased progesterone is the primary mechanism that triggers the onset of labor.