What is oxytocin - Biology?

oxytocin is a hormone which plays a major role in human birth by causing vigorous contraction in the muscles.

Oxytocin does more than just one thing. It is true that as the cervix begins to dilate at the beginning of labor, oxytocin is put out in larger and larger amounts to increase the intensity of the smooth muscle contractions of the uterus to propel the child through the vagina in childbirth.
However, oxytocin does other things related to reproduction too. It is oxytocin that stimulates milk production in the mammary glands of the pregnant mother, and in response to the suckling of the child, oxytocin is released to allow what is called "milk let-down", which is a contraction of the milk glands under the armpits that sends milk into the milk ducts and to the nipples. This let-down reflex is so sensitive that in newly nursing mothers, even the cry of the baby will stimulate milk to begin to be secreted from the nipples because the brain perceives that the child is hungry and stimulates the pituitary gland to release oxytocin. This is why new mothers who are breastfeeding wear absorptive pads in their nursing bras for the first month or so, until the feedback response levels out. If they don't, they can leak milk through their shirts. Trust me, I've been there.
Even a warm shower can stimulate milk let-down in the early weeks, so sensitive is the mother's body to oxytocin release. Eventually, the supply and demand become synchronized and it becomes a wonderful system, as the mom makes milk according to how often her child suckles and the involuntary let-down of milk happens only when the baby begins to feed.
Oxytocin also helps a nursing mom get rid of her pregnancy belly faster because it still causes contraction of the uterus which helps to shrink the uterus back to normal faster than if the mom is bottle feeding, so that's another bonus of nursing your child.

Oxytocin is a peptide hormone the brain produces in the posterior pituitary. It appears to play many roles in our lives, starting with birth, when it manifests one of the few examples of positive feedback during labor.