Why is menstrual flow a sign that girl can be pregnant?

Menstrual flow is not a sign of pregnancy. In fact, it is a sign that a woman is not pregnant. Menstruation occurs when the lining of the uterus (the endometrium) sheds and is expelled from the body through the vagina. This process usually begins around 14 days after ovulation and lasts for 3-7 days. If a woman becomes pregnant, the fertilized egg implants in the endometrium and begins to develop into a fetus. This prevents the endometrium from shedding, so menstruation does not occur.