Answers to Frequently Asked Questions About...
Risk of Pregnancy
If I have what appears to be a normal period, can I still be pregnant?
It is rare for a woman to have what appears to be a normal period, but still be pregnant. 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, in sharp contrast to the bleeding reported with ordinary menstrual periods, for which women in the study typically used 4-8 pads on the heaviest days. Only one woman experienced bleeding that was similar in length and intensity to usual menses: 5 consecutive days, and at most only three pads or tampons were used per day. 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.