Introduction: Little is known about the occurrence and patterns of vaginal bleeding during the earliest stages of pregnancy. We explore this in a prospective study of early pregnancy.

Methods: A total of 221 healthy women kept daily diaries and provided daily urine samples while trying to become pregnant. Of these, 151 women became clinically pregnant [i.e. pregnancy that lasted >/=6 weeks beyond last menstrual period (LMP)] during the study. Diaries provided information on days with vaginal bleeding and sexual intercourse. Urine hormone assays were used to identify ovulation and implantation. Women were interviewed about their medical histories and lifestyle factors.

Results: A total of 14 women (9%) recorded at least 1 day of vaginal bleeding during the first 8 weeks of pregnancy. Twelve of these 14 pregnancies continued to a live birth. Bleeding tended to occur around the time when women would expect their periods, although rarely on the day of implantation. Bleeding was not associated with intercourse.

Conclusions: Early bleeding in clinical pregnancies is generally light, and not likely to be mistaken for LMP. Thus, early bleeding is unlikely to contribute to errors in LMP-based gestational age. We found no support for the hypothesis that implantation can produce vaginal bleeding. Similarly, intercourse did not cause bleeding. Nearly all women with bleeding went on to have successful pregnancies.