A menstrual period occurs when the uterus sheds its lining, while implantation bleeding occurs when a fertilized egg burrows into the uterine lining. A menstrual period takes place when a woman is not pregnant, while implantation bleeding can be one of the earliest signs of pregnancy.
According to the Mayo Clinic, implantation bleeding takes place approximately 10 to 14 days after conception. It is much lighter than a period, and it only occurs for 2 to 3 days. Implantation bleeding occurs around the time that a woman expects her period. As a result, some women mistake implantation bleeding for their period.
The uterine lining thickens in preparation for a possible pregnancy. If there is no pregnancy, this lining must be discarded via menstruation. A period lasts for 3 to 5 days on average and is heavier at the beginning.