Background: The reliability of retrospective time to pregnancy (TTP) has been established, but its validity has been assessed in only 1 study, which had a short follow-up.
Methods: Ninety-nine women enrolled a decade earlier in a prospective TTP study were queried by means of mailed questionnaires about the duration of time they had required to become pregnant. Their responses were compared with their earlier data from daily diaries (gold standard).
Results: One-third of women could not recall their earlier TTP either in menstrual cycles or calendar months. Only 17%–19% of women recalled their TTP exactly. Agreement increased to 41%–51%, 65%–72%, and 72%–77% when defined as ±1, ±2, and ±3 months, respectively. Women with longer observed TTPs or previous pregnancies were more likely to under-report their TTP.
Conclusions: The findings raise questions about the commonly assumed validity of self-reported TTP. Recalled TTP may introduce error when estimating fecundability or classifying couples’ fecundity status.