Like the menstrual cycle, the period may have slight variations in individual women. Bleeding is usually heavier in the first day or two and then becomes lighter for the next two or three days. FSH stimulates the ovary to produce estrogen in the first half of the menstrual cycle. Estrogen causes a thickening of the lining of the uterus. In mid-cycle, a sudden increase in LH causes ovulation, and progesterone alters the nature of the uterus lining in preparation for a fertilized egg.
Fertilization produces an embryo that stimulates another hormone to maintain the uterus lining.
If fertilization does not take place, the uterus lining is shed as the menstrual flow and the cycle of events begins all over again. The pituitary gland then releases LH, controlled by feedback of progesterone from the ovaries. The feedback of the various hormones is detected by the hypothalamus in the brain. The hypothalamus can also be affected by other factors, such as emotions, anxiety, or depression, and the effects of other hormones in the bloodstream. In neither case is the exact mechanism fully understood.