Background: Despite the widespread use of retrospectively reported time to pregnancy to evaluate fertility either as an outcome or as a risk factor for chronic disease, only two small studies have directly compared prospective data with later recall.
Methods: The North Carolina Early Pregnancy Study (1982-1986) collected prospective time-to-pregnancy data from the beginning of participants' pregnancy attempt. In 2010, (24-28 years later) women were sent a questionnaire including lifetime reproductive history that asked about all prior times to pregnancy. Of the 202 women with prospective time-to-pregnancy data, 76% provided recalled time to pregnancy.
Results: A lower proportion of women with times to pregnancy ≥3 cycles provided a recalled time to pregnancy than women with times to pregnancy <3 cycles. Also, high gravidity or parity was associated with a lower likelihood of providing a recalled time to pregnancy. Women with very short or very long times to pregnancy (1 cycle or ≥13 cycles) had good recall of time to pregnancy. Positive predictive values of 1 or ≥13 cycles were 73% and 68%, respectively, while positive predictive values for other categories of time to pregnancy ranged from 38% to 58%. The weighted kappa statistic for recalled versus prospective time to pregnancy was 0.72 (95% confidence interval: 0.65, 0.79).
Conclusions: Recalled time to pregnancy showed good agreement with prospective time to pregnancy. Informative missingness must be considered when imputing recalled time to pregnancy. Associations observed in future studies can be corrected for misclassification.