Pregnant women can't have periods, but spotting around the time a period is due is not uncommon. Implantation bleeding occurs 10 to 14 days after conception, which is usually around the time a woman would expect to have her period.
According to the Mayo Clinic, implantation bleeding is normal, but not every pregnant woman will experience it. The bleeding is usually lighter than a period and only lasts a short time. It occurs when the fertilized egg attaches to the wall of the uterus. Some women do mistake this type of bleeding for a period, which can sometimes lead to an incorrect due date.