All women experience a kind of hormone "withdrawal" after delivery of a child, as estrogen and progesterone levels—which rise as much as times above normal during pregnancy—now drop sharply to levels far below normal.

Explanation:

Pregnancy is a type of the condition in the female in which one embryo is formed and developed in the uterus. There are many hormones that plays important role during the pregnancy period. Estrogen and progesterone hormones are very important hormones that plays an important role in this process. Estrogen makes the uterus and placenta able for the process of vascularization. Progesterone is important for the lining and thicking of the endometrium. So the level of these hormones increases about 50 times more during the pregnancy period.

the answer to your question is sperm