Answer:
When the critical hormone oxytocin is released in high enough concentration by the mother's pituitary gland, the uterus begins periodic contractions
Step-by-step explanation:
The oxytocin is a polypeptide hormone produced in the hypothalamus and corpus luteum, so the nervous system controls its secretion. It is known to participate in feminine reproduction stimulating uterine contractions during labor and milk ejection. It acts contracting the myometrium and myoepithelial cells.
The oxytocin also takes place in kidneys and cardiovascular systems.
Its receptors are located in the uterus, mammal gland, brain, kidney, heart, bones, endothelial cells, and ovary tissue.
Synthetic oxytocin is administrated to pregnant women to induce or accelerate labor when they can not secrete it by themselves.