Every month a normally ovulating female releases a mature egg according to precise timing governed by hormones.
The hypothalamus is a small gland located at the base of the brain, which can be thought of as a "thermostat". Estrogen levels are monitored by the hypothalamus and as they increase, the hypothalamus signals the pituitary to reduce production of FSH. Once the hormone levels indicate that the follicles are mature, the hypothalamus signals the pituitary to release a surge of luteinizing hormone (LH). The spike in LH levels triggers ovulation approximately 36 hours after the surge. The ovaries begin to produce progesterone, which also supports the development of the endometrium. Rising levels of human chorionic gonadotropin (hCG), produced by the placenta, are an indication of pregnancy. If pregnancy does not occur, the lining of the endometrium breaks down and is released during menstruation.
Lack of ovulation is termed "anovulation" and irregular ovulation is termed "oligoovulation".