The female reproductive organs include the vagina, cervix, uterus, fallopian tubes and ovaries. The lining of the uterus is called the endometrium.

Before ovulation, the endometrium grows and becomes thick: the endometrial tissue and blood vessels support the fertilized egg when it enters the uterus.

During ovulation, an egg is released from one of the ovaries and enters the fallopian tube. If the egg is fertilized as it passes through the fallopian tube, it attaches to the endometrium on the inside of the uterus. The endometrium continues to support the fertilized egg as it grows.

If the egg is not fertilized, the egg does not attach to the endometrium and the endometrium breaks down. The uterus sheds the endometrium, which causes the bleeding associated with the menstrual period.