Why does menstruation stop during pregnancy?

Menstruation stops during pregnancy for a couple of reasons. First, the hormones that the corpus luteum (the place on the ovary where the egg came from) secretes include progesterone which stops the follicle stimulating hormone. Follicle stimulating hormone (FSH) is what makes your ovary grow eggs and it stops because of the progesterone. Second, when the follicle stimulating hormone is not present, a new egg will not grow. Without a new egg to grow, ovulation will not occur and progesterone will not drop so your lining will stay put until you give birth.