What is the hormone that aids in parturition and causes milk let down?

Oxytocin is the hormone that plays a crucial role in both parturition (the process of giving birth) and milk let-down in mammals. During parturition, oxytocin is released in response to the stretching of the cervix and the pressure from the fetus, facilitating uterine contractions that help with the delivery of the piglets. Following delivery, oxytocin promotes the contraction of myoepithelial cells in the mammary glands, which assists in the expulsion of milk during nursing.

Understanding the mechanisms of oxytocin’s actions provides insight into its essential role in reproductive physiology, especially in swine where effective lactation and timely parturition are vital for the health and growth of the offspring. The choices related to prolactin, estrogen, and testosterone do have their specific functions in reproductive biology but do not directly pertain to both parturition and the immediate reflex of milk let-down in the manner that oxytocin does.