When the critical hormone is released in high enough concentration by the mother's pituitary gland, the mother's uterus begins periodic contractions?

Oxytocin

When the critical hormone oxytocin is released in high enough concentration by the mother's pituitary gland, the mother's uterus begins periodic contraction.

Oxytocin is a peptide hormone produced by the hypothalamus and released into the blood stream by the posterior pituitary. Oxytocin works on certain organs in the body such as the breast and uterus. Oxytocin influences important functions of the reproductive system, including causing contraction of the cervix and uterus during labor, lactation, and sexual reproduction.

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