The menstrual cycle
After puberty the uterus goes through a regular cycle of changes known as the menstrual cycle, where ova (eggs) are released, and the womb lining thickens to receive and implant a fertilized egg. The cycle is an integral part of the reproductive mechanism.
First, hormones stimulate a thickening of the lining of the uterus (called the endometrium) and an increase in its blood supply.
Then, under cyclic hormonal influence, one of the two ovaries is stimulated to release an egg (an ovum) about once a month. This is called ovulation.
The egg then starts its journey along the Fallopian tube to the uterus.
If the egg is not fertilized within about 48 hours of its release, hormonal stimuli are withdrawn, resulting in the disintegration of the endometrium. As the womb lining separates from the wall to which it is attached, it is discharged through the vagina.
Some bleeding occurs, which is called menstruation or the menstrual period.