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

oxytocin

Explanation:

Oxytocin is a hormone naturally produced by our body to generate contractions of the uterus during labor and the release of milk during breastfeeding. It was synthesized in the laboratory and thus became a very useful medication that can help save lives if correctly indicated.

This hormone can also be used to initiate labor when it does not occur naturally, and can regulate contractions when they are not effective and it is diagnosed that labor is not progressing properly.

In addition, it can also be used to contract the uterus after childbirth or abortion when bleeding is very abundant, preventing bleeding that could lead to loss of the uterus and even death if not properly controlled.

what's the question?

the unequal cytoplasmic division that  results  in  one egg  and three polar bodies ensures that a fertilized  egg has  ample nutrients  forits journey  to  the uterus.