The American Pregnancy Association reports that it is not possible for a pregnant woman to menstruate following conception. However, some fluid discharge or spotting is possible.
The Office on Women's Health explains that menstruation occurs when a woman's body sheds its uterine lining. The lining grows during the first half of the menstrual cycle, preparing the body for possible pregnancy. Halfway through the cycle, the ovaries release an egg, which travels through the fallopian tubes to the uterus. If the egg is fertilized, the thickened uterine lining remains and provides nourishment to the embryo. If the egg is not fertilized, the lining is shed.