It is not possible for a woman to menstruate when she is pregnant. However, it is not uncommon for women to experience bleeding during pregnancy, particularly during the first trimester. According to the Mayo Clinic spotting or light implantation bleeding between 10 and 14 days after conception is reasonably common.
According to Baby Center, menstruation can only happen when a woman is not pregnant. During pregnancy, the lining of the uterus remains intact to enable the foetus to grow. It does not shed until the baby has been born.
Heathline explains that some women experience implantation bleeding shortly after conception when a fertilized egg implants into the lining of the uterus. This bleeding usually occurs around the time of the woman's next expected period and is sometimes mistaken for menstruation.
The Mayo Clinic advises women to contact their health care provider if they have concerns about any vaginal bleeding during pregnancy.