Which hormone is primarily responsible for initiating labor?

Oxytocin is the hormone primarily responsible for initiating labor. It plays a crucial role in prompting uterine contractions, which are essential for the progression of labor. During the onset of labor, the body's levels of oxytocin increase, stimulating the smooth muscles of the uterus to contract. These contractions help in the dilation of the cervix and the eventual delivery of the baby.

While other hormones mentioned, such as prolactin, estradiol, and progesterone, have important roles in pregnancy and lactation, they do not initiate labor in the same direct manner as oxytocin. Prolactin primarily supports milk production after birth, estradiol is involved in preparing the uterus and regulating many aspects of reproduction, and progesterone helps to maintain pregnancy by inhibiting uterine contractions. However, as labor nears, the balance shifts in favor of oxytocin, making it the key hormone that drives the labor process.