Why are menstrual cycles absent during pregnancy ?

The menstrual flow results due to break down 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 fertilised.

During pregnancy all these stages / phases of menstrual flow stop and corpus leuteum secretes projesterone in large amount which is necessary for care of endometrium. These changes don't let menstrual flow occur during pregnancy. Lack of menstruation may be indicative of pregnancy. However it may also be caused due to some other underlying cause like stress, poor health.