Why does your menstruation stop when a woman is pregnant?

Menstruation is what happens when a woman doesn't get pregnant - after ovulation the uterine lining builds-up in preparation for possible pregnancy, if a woman doesn't get pregnant then menstruation occurs to shed the uterine lining so it can start fresh next cycle. During pregnancy the uterine lining is busy keeping the growing embryo/fetus safe, and women no longer ovulate.