Contraceptive failure related to estimated cycle day of conception relative to the start of the last bleeding episode

BackgroundThe objective of this study was to estimate the menstrual cycle day of conception in women presenting for abortion.Study DesignThis was a retrospective chart survey in two urban free-standing abortion clinics.ResultsThere were 913 charts reviewed of women presenting for an abortion at less than 63 days' gestation as determined by endovaginal ultrasound who were “sure” of the date of their last normal menstrual period. The estimated mean cycle day of conception determined by sonographically estimating length of gestation was 14.6. There were 26 (26.3%) of 99 women using cyclic hormonal contraception who conceived before 10 days after the onset of withdrawal bleeding compared to 100 (14.7%) of 679 who conceived before 10 days after the onset of their last menstrual period who were using all other forms of contraception, including “none” (p=.005). No other differences in the proportions conceiving early in the cycle were observed with respect to age, ethnicity or obesity.ConclusionThese data suggest that there is a sizeable subset of women who ovulate earlier after onset of withdrawal bleeding when using 21/7 hormonal contraceptives than after onset of menses when not using hormonal contraception. It is possible that women using hormonal contraceptives may have a higher risk of pregnancy if they ovulate sooner after the onset of bleeding.