Clustering of retrospectively reported and prospectively observed time-to-pregnancy.

Purpose: Given reportedly high clustering but limited validity of retrospectively reported time-to-pregnancy (TTP), we assessed within-woman clustering for retrospectively reported TTPs alone and including gold-standard prospectively observed TTPs among women with 2 or more retrospectively reported and 1 or more prospectively observed TTPs. We further investigated whether past trying times inform future trying time among women with 1 or more retrospectively reported and 1 or more prospectively observed TTPs.

Methods: Five hundred one couples attempting pregnancy were prospectively observed until human chorionic gonadotropin pregnancy or 12 months of trying. Women reported TTP for past planned pregnancies. Clustering as measured by the frailty variance was estimated using discrete Cox frailty models, adjusted for age, body mass index, smoking at each attempt. Utility of past attempts to inform future attempts was assessed with discrete Cox models and relative risk regression, adjusted for enrollment age, body mass index, smoking.

Results: Seventy-five women with 2 or more prior pregnancies contributed 180 retrospective and 91 prospective TTPs for frailty modeling. Retrospectively reported TTP clustering was high (frailty variance = 0.89) but substantially lower when including prospectively observed TTPs (frailty variance = 0.42). Among 202 women with 1 or more prior pregnancies, past trying times did not inform future trying time.

Conclusions: TTP recall rather than TTP may account for clustering. Past trying times may not inform future trying times.