Is a substance released by the pituitary gland late in pregnancy and causes the uterine contractions late in the birth process that are necessary to push out the fetus.

Oxytocin.

Explanation:

Oxytocin is a polypeptide hormone that are released from the posterior pituitary. Oxytocin is released directly into the blood and was first discovered by  Henry Dale.

The main function of oxytocin is the uterine contraction in the late pregnancy that helps in the expel of fetus from the mother's body. Oxytocin also plays an important role in the sexual behavior and social bonding. The oxytocin is controlled by the positive feedback mechanism in the body.

Thus, the answer is oxytocin.

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