What triggers menses?

Menses, or a period, is the shedding of the lining of the uterus (womb) through the vagina. It occurs when a fertilized egg does not implant in the uterus and levels of the hormones estrogen and progesterone drop. This drop in hormone levels causes the lining of the uterus to break down and be shed through the vagina. This process typically lasts for about three to seven days.