During the reproductive years, a woman will release one egg each month. The period starts when the blood level of female hormones called estrogen and progesterone drop. The pituitary gland then starts producing follicle stimulating hormone (FSH) which makes several follicles grow. The follicles contain eggs. As the follicles grow, they produce estrogen which thickens the lining of the uterus.
Eventually, one follicle will become dominant and grow to maturity while the others stop growing. When the egg in the dominant follicle is mature and the estrogen levels peak, luteinizing hormone (LH) is released and ovulation occurs. This means that the egg is released from the ovary.