It is rare, but possible, for a woman to menstruate when she is pregnant. In a very few cases, a woman’s menstrual cycle may continue throughout the first trimester of pregnancy.
It is more likely that bleeding in the early months of pregnancy has a cause other than menstruation. Light bleeding sometimes occurs when a fertilized egg is implanted in the uterus. Bleeding is also sometimes a symptom of a problem with the pregnancy.
For example, in an ectopic pregnancy, the egg implants outside the womb and causes bleeding. At times, blood flow is an indication of a miscarriage. In any case, bleeding during pregnancy should be checked out by a doctor.
Some women do have vaginal bleeding during pregnancy. Some even report intermittent bleeding that seems like a regular period to them. But vaginal bleeding during pregnancy is not the same thing as menstruation.
Menstruation only happens when you’re not pregnant: Each month, your uterus grows a thick blood-rich lining in preparation for an egg to embed there. If you don’t get pregnant that month, you shed this tissue and blood – that’s your menstrual period.
But once an egg embeds in the uterine lining, hormones tell the blood-rich tissue to stay intact to support the growing baby. And you won’t shed it and start having your period again until your pregnancy is over.