It's not likely for a woman to be pregnant and still menstruate -- especially past very early pregnancy -- though some women will experience some vaginal bleeding (which is not the same as menstruation, or a period) during pregnancy.
Part of the problem is that many people term any vaginal bleeding at all as a period, which isn't accurate.
A woman’s period is caused by her whole menstrual cycle. During the menstrual cycle, the body releases hormones that send signals causing the ovaries to release an egg and the lining of the uterus will become thicker to allow implantation to occur if the egg is fertilized. If the egg is not fertilized, the uterine lining and the egg are shed, this bleeding considered a woman’s menstruation or period.
During pregnancy, however, a woman’s body begins to function in a way to allow the baby to develop in a safe environment. The ovaries will stop releasing eggs monthly (otherwise women could become pregnant more than once while pregnant), and the uterus doesn’t release the lining but rather makes a home for the growing baby. Hormones (which normally drop) will continue to rise throughout the nine months of pregnancy.
There are a number of reasons that some bleeding may occur during pregnancy, and actually many of them present little health risk to the mother or the baby. Implantation bleeding, for instance, is something that can happen for some women: that's bleeding that can occur when the fertilized egg is implanting in the uterine wall. While that bleeding tends to be lighter than a period, because it can happen around the same time a period is expected, some women can easily confuse that for a menstrual period. There is also something called decidual bleeding, and it’s actually one of the most common causes of period-like bleeding that occurs during pregnancy. Because hormones can be very discombobulated during pregnancy, part of the lining of the uterus can be shed. Usually it’s not considered a health risk, but if a woman doesn’t know she’s pregnant it can seem like a period.
Sometimes bleeding can mean a complication, which is why being open and honest with the doctor is very important. Light spotting during pregnancy does happen often enough, but spotting for many isn't a normal period. If pregnancy is suspected or there's been a risk and a period is missed or only light spotting occurs, pregnancy isn't generally ruled out right away.
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