There are some women who are trying to get pregnant who assume that they are not pregnant because they start bleeding right around when they are supposed to get their period. There are instances when this is just a sign of implantation bleeding. This is a type of bleeding that occurs when a fertilized egg cell moves to the uterus in order to start growing.
Menstruation bleeding is different. It involves the death of an unfertilized egg and this constantly happens every month. You can tell the difference between the two by checking the type of blood that is released. Is the blood brownish or bright red? The menstrual period is usually bright red while bleeding from implantation might be a bit on the brownish side.