A human female is born with a million or so immature eggs, or oocytes, within each of her ovaries. These oocytes are arrested at an early stage of meiosis. Most of these oocytes will die before they ever have a chance to mature, and by the time a woman reaches sexual maturity, each ovary contains about 200,000 oocytes. Every month, a dozen of these oocytes re-initiate their development, but usually only one of them makes it to full maturity.
The cycle in which an oocyte matures, erupts from the ovary, and then travels down the oviduct to the uterus is called the ovarian cycle. The cycle varies in length, lasting on average 28 days. The ovarian cycle is tightly coordinated with the uterine cycle, in which the lining of the uterus grows and prepares for an embryo to implant. If, by the end of the cycle, the oocyte has not been fertilized and an embryo has not implanted, then the lining of the uterus sloughs off in a process called menses, or menstruation.
The chart on the left summarizes the roles of hormones in controlling the ovarian and uterine cycles. In addition to luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, and progesterone, there is yet another hormone that plays an important role in these cycles. Called gonadotropin-releasing hormone (GnRH), this hormone is released from the hypothalamus. GnRH triggers the anterior pituitary to release LH and FSH.
Most of these hormones have clear effects:
The effect of estrogen and progesterone on the brain is not as straightforward, however. During the few days prior to ovulation, these ovarian hormones have a stimulatory effect. The rest of the time, these hormones inhibit the hypothalamus and anterior pituitary from releasing GnRH, LH, and FSH.
A pregnancy puts the 28-day cycle on hold. When a multicelled embryo, called a blastocyst, implants in the uterine lining, a new hormone controls the cycle. This one, called human chorionic gonadotropin (hCG), is secreted by a layer of cells surrounding the blastocyst. hCG is similar in structure to LH and, like LH, it maintains the function of the corpus luteum in the ovary. The corpus luteum continues to release estrogen and progesterone, which, in turn, continue to inhibit the brain from releasing GnRH, LH, and FSH. Without LH and FSH, the pregnant woman will not ovulate.
Textbook Reference: Concept 37.2 The Mammalian Reproductive System Is Hormonally Controlled