The menstrual cycle is a cycle in women during which the body prepares for pregnancy. The walls of the uterus thicken with blood to prepare to receive a fertilised egg which will grow into an embryo. The thickening of the uterine walls occurs in the first half of the cycle - which lasts on average about 14 days. Ovulation, release of a fertile egg from the ovary takes place on about day 14 and then, if it is fertilised, implantation in the wall of the uterus takes places a few days later.
If no pregnancy occurs, the uterine lining is shed in menstruation, which starts on about day 28 of the cycle. The cycle then reverts to day one.
During the cycle, changes occur in the concentrations of follicle stimulating hormone, lutenising hormone, oestrogen and progesterone. FSH and LH are at the peak levels at the time of ovulation; oestrogen is also at its highest level at that time. These three hormone levels then tail off for the second half of the cycle but the hormone progesterone builds up, reaching its peak level just before menstruation begins.
If no pregnancy occurs, the uterine lining is shed in menstruation, which starts on about day 28 of the cycle. The cycle then reverts to day one.
During the cycle, changes occur in the concentrations of follicle stimulating hormone, lutenising hormone, oestrogen and progesterone. FSH and LH are at the peak levels at the time of ovulation; oestrogen is also at its highest level at that time. These three hormone levels then tail off for the second half of the cycle but the hormone progesterone builds up, reaching its peak level just before menstruation begins.