Pre-gravid oral contraceptive use and time to pregnancy: a Danish prospective cohort study.

STUDY QUESTION Is there an association between oral contraceptive (OC) use (age at the start of use, duration of use, ethinylestradiol dose and generation) and time to pregnancy (TTP)? SUMMARY ANSWER Although OC use was associated with a transient delay in the return of fertility, we found no evidence that long-term OC use deleteriously affects fecundability. WHAT IS KNOWN ALREADY Studies using retrospective data on TTP have reported a short-term delay in the return of fertility after OC use. However, little is known about the long-term OC use and TTP. STUDY DESIGN, SIZE, DURATION Data were derived from 'Snart Gravid.dk', a prospective cohort study that enrolled participants from 1 June 2007 to 31 May 2010. The final study population consisted of 3727 women. PARTICIPANTS, SETTING, METHODS Eligible women were Danish pregnancy planners, aged 18-40 years, who completed a baseline questionnaire and bimonthly follow-up questionnaires until conception or for 12 months, whichever came first. Cohort retention was 80%. We used proportional probability regression models to estimate fecundability ratios (FRs) and 95% confidence intervals (CIs), with adjustment for potential confounders. MAIN RESULTS AND ROLE OF CHANCE Compared with barrier methods, the use of OCs as the last contraception method before attempting to conceive was associated with a short-term delay in return of fertility (FR = 0.87, 95% CI: 0.79-0.96). Longer term OC use was associated with higher fecundability: compared with OC use for less than 2 years; FRs were 0.98 (95% CI: 0.83-1.15) for 2-3 years, 1.16 (95% CI: 0.98-1.37) for 4-5 years, 1.10 (95% CI: 0.93-1.29) for 6-7 years, 1.17 (95% CI: 0.99-1.38) for 8-9 years, 1.23 (95% CI: 1.04-1.46) for 10-11 years and 1.28 (95% CI: 1.07-1.53) for ≥12 years of OC use. LIMITATIONS, REASONS FOR CAUTION Because this was a non-experimental study, where study volunteers provided information about their history of contraceptive use at baseline and were followed prospectively to assess their waiting times to pregnancy, there was some potential for error in the reporting of OC use and TTP. Nevertheless, participants reported data on OC use before the occurrence of pregnancy, thereby reducing the potential for systematic bias. WIDER IMPLICATIONS OF THE FINDINGS Women who have used OCs for many years should be reassured as there was no evidence that long-term OC use has a deleterious affects on fecundability. Both short- and long-term OC users are likely to experience a transient delay in conception compared with those discontinuing barrier methods. STUDY FUNDING This study was supported by the National Institute of Child Health and Human Development (R21050264) and the Danish Medical Research Council (271-07-0338). The authors declare that there are no conflicts of interest.