The menstrual cycle is the process that prepares the uterus for pregnancy. The average length of the menstrual cycle is 28 days, however in reality the normal menstrual cycle may vary in length from 26-35 days.
Menstrual cycles have two phases.
The first, or proliferative phase, may vary from 13-20 days and ends when ovulation occurs. This phase varies from one woman to another and may also differ from month to month. The second, or secretory phase, begins after ovulation and ends when the period starts. This phase generally lasts 14 days for all women. Menstruation is controlled by a complex series of hormonal interactions between the thyroid, adrenal and pituitary glands, hypothalmus and the ovaries.