Why are menstrual cycles absent during pregnancy?

Explanation

The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertillised.
During pregnacy, all events of the menstrual cycle stop and the corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the endometrium. These changes lead to no menstruation, during pregnancy.
Not Lack of menstruation may be indicative of pregnancy. However, it may also be cauesd due to some other underlying causes like stress, poor health, etc.