The Normal Menstrual Cycle
During the menstrual cycle, two hormones, called estrogen and progesterone, are made by the ovaries. Each month, these hormones cause the endometrium to grow in preparation for a possible pregnancy. About 12–14 days before the start of the period, an egg is released from one of the ovaries. This is called ovulation. The egg then moves into one of the fallopian tubes. There it can be fertilized by a sperm. If it is not, pregnancy does not occur. The levels of hormones decrease. This decrease is a signal for the uterus to shed its lining. This shedding is the menstrual period.
The cycle begins with the first day of bleeding of one period and ends with the first day of the next. In most women, this cycle lasts about 28 days. Cycles that are shorter or longer by up to 7 days are normal.