It is not possible to menstruate while pregnant, but the American Pregnancy Association notes that some women experience light bleeding or spotting during pregnancy. However, this bleeding is not as heavy or as regular as a typical period. Any bleeding during pregnancy should not be enough to fill pads or tampons over a few days, and any bleeding to this degree can be a sign of miscarriage or ectopic pregnancy.
A period is the shedding of the uterine lining, and because this does not happen during pregnancy, a woman's periods cease during pregnancy, according to Parents magazine. Although light bleeding during the first month of pregnancy while the fertilized egg is implanted in the uterine lining is common, a doctor should always be consulted if any bleeding occurs during pregnancy. If implantation is the cause of bleeding, it carries little risk, may be accompanied by light cramping and occurs only at the beginning of the pregnancy and not throughout the entire term.
During the first trimester, bleeding can indicate an impending miscarriage or an ectopic pregnancy, but both of these are often accompanied with cramping, according to Parents magazine. If a woman is pregnant and passing a lot of blood, especially with large clots, it is important to contact a doctor immediately or visit an emergency room.