Early in the menstrual phase, before menstruation begins, the pituitary secretion increases by FSH (Follicle Stimulating Hormone), which stimulates ovarian growth (follicles). The ovaries grow to form estrogen (female sex hormone).
As estrogen levels rise, the secretion of FSH from the pituitary gland decreases, as part of what is called the feedback regulation between the pituitary and ovaries. At the same time, estrogen causes an increase in the thickness of the uterine lining, so that the uterus is ready to receive a fertilized egg. When the ovary is ready for ovulation, its high estrogen production triggers the pituitary gland to secrete the ovulation hormone LH (Luteinizing Hormone).
The elevated LH level causes ovulation. The follicle from which the egg came is developed into the yellow body. The yellow body secretes the hormones estrogen and progesterone. Progesterone completes the uterine mucosa so it is ready for a fetal system to establish in it.
If the woman does not become pregnant, the yellow body perishes 12-14 days after ovulation and production of estrogen and progesterone ceases. Menstruation will then occur and a new cycle will start.