Willis SK, Hatch EE, Wesselink AK, Rothman KJ, Mikkelsen EM, Ahrens KA, Wise LA. Post-partum interval and time to pregnancy in a prospective preconception cohort. Paediatr Perinat Epidemiol. 2021 May;35(3):271-80. doi: 10.1111/ppe.12702

BACKGROUND: Little is known about the influence of the post-partum interval-defined as the time between giving birth and attempting to conceive again-on subsequent fecundability.

OBJECTIVES: We evaluated the association between the post-partum interval and fecundability in Pregnancy Study Online (PRESTO), a web-based prospective preconception cohort of pregnancy planners from the United States and Canada.

METHODS: Eligible women were aged 21-45 years, attempting pregnancy, and not using fertility treatment. Women completed a baseline questionnaire to ascertain information on demographics, life style factors, and reproductive history, including detailed information on all previous pregnancies. They completed bi-monthly follow-up questionnaires for up to 12 months to update pregnancy status over time. We used proportional probabilities regression models to estimate fecundability ratios (FRs) and 95% confidence intervals (CIs) adjusted for sociodemographic and reproductive history covariates. Analyses were restricted to multiparous women who had been attempting pregnancy with the same male partner for ≤6 menstrual cycles at enrolment.

RESULTS: During 2013-2019, 1489 female participants contributed 959 pregnancies and 5003 cycles. The median post-partum interval was 18 months. Compared with a 12- to 23-month post-partum interval, FRs for post-partum intervals of <12, 24-47, and ≥48 months were 0.89 (95% CI 0.77, 1.04), 1.06 (95% CI 0.91, 1.23), and 0.81 (95% CI 0.62, 1.05), respectively. When restricting to women without a history of subfertility, results were consistent for long post-partum interval and attenuated for short post-partum interval.

CONCLUSION: Among North American pregnancy planners, long post-partum intervals (≥48 months) were associated with slightly reduced fecundability. Short post-partum intervals (<12 months) were weakly associated with reduced fecundability in some subgroups including women with a history of caesarean delivery and planned pregnancies.

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