Answers to Frequently Asked Questions About...
Risk of Pregnancy
Is implantation a cause of bleeding?
It appears that implantation is rarely, if ever, a cause of bleeding. One prospective study examined vaginal bleeding in early pregnancy. Nine per cent of women with clinical pregnancies (14/151) reported at least one day of bleeding. Bleeding was typically light, requiring only one or two pads or tampons in 24 hours. Only one episode of bleeding occurred at implantation; most bleeding began at least five days after implantation.
Source: Harville EW, Wilcox AJ, Baird DD, Weinberg CR. Vaginal bleeding in very early pregnancy. Human Reproduction 2003;18(9):1944-7.
More details on these studies and bleeding patterns overall can be
found in this thorough and up-to-date academic
review 
of the medical and social science literature on emergency contraception.