Although it is impossible to be pregnant if while menstruating, many women have light bleeding during their pregnancy, especially during the first trimester. This bleeding may be mistaken for a menstrual period. A common cause of bleeding during the first trimester is implantation bleeding.
According to the Mayo Clinic, implantation bleeding occurs when the fertilized egg becomes attached to the lining of the uterus and can happen around the time that women would be expecting their next period. This bleeding is generally light and often goes unnoticed. In other cases, it may be heavy enough to be mistaken for a period. Implantation bleeding will generally resolve on its own and doesn't require any treatment.
Generally, vaginal bleeding during the first trimester is not a cause for concern. However, for people who experience bleeding that persists or worsens, clots or tissue passing from the vagina, cramps or back pains, or any fever and chills, they need to contact their health care provider immediately. These symptoms may be a sign of an ectopic pregnancy, infection or impending miscarriage. As a pregnancy progresses into the second and third trimesters, any sign of bleeding, even light bleeding, may be cause for concern, so it's important for women to let their doctors know as soon as possible.