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The Curse of the Perinatal Epidemiologist: Inferring Causation Amidst Selection

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Abstract

Purpose of Review

Human reproduction is a common process and one that unfolds over a relatively short time, but pregnancy and birth processes are challenging to study. Selection occurs at every step of this process (e.g., infertility, early pregnancy loss, and stillbirth), adding substantial bias to estimated exposure-outcome associations. Here, we focus on selection in perinatal epidemiology, specifically, how it affects research question formulation, feasible study designs, and interpretation of results.

Recent Findings

Approaches have recently been proposed to address selection issues in perinatal epidemiology. One such approach is the ongoing pregnancies denominator for gestation-stratified analyses of infant outcomes. Similarly, bias resulting from left truncation has recently been termed “live birth bias,” and a proposed solution is to control for common causes of selection variables (e.g., fecundity, fetal loss) and birth outcomes. However, these approaches have theoretical shortcomings, conflicting with the foundational epidemiologic concept of populations at risk for a given outcome.

Summary

We engage with epidemiologic theory and employ thought experiments to demonstrate the problems of using denominators that include units not “at risk” of the outcome. Fundamental (and commonsense) concerns of outcome definition and analysis (e.g., ensuring that all study participants are at risk for the outcome) should take precedence in formulating questions and analysis approaches, as should choosing questions that stakeholders care about. Selection and resulting biases in human reproductive processes complicate estimation of unbiased causal exposure-outcome associations, but we should not focus solely (or even mostly) on minimizing such biases.

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Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance

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Funding

Dr. Snowden is supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (grant number R00 HD079658-03).

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Correspondence to Jonathan M. Snowden.

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Jonathan M. Snowden reports grants from NICHD during the conduct of the study. Marit Bovbjerg, Mekhala Dissanayake, and Olga Basso each declare no potential conflicts of interest.

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This article does not contain any studies with human or animal subjects performed by any of the authors.

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This article is part of the Topical Collection on Reproductive and Perinatal Epidemiology

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Snowden, J.M., Bovbjerg, M.L., Dissanayake, M. et al. The Curse of the Perinatal Epidemiologist: Inferring Causation Amidst Selection. Curr Epidemiol Rep 5, 379–387 (2018). https://doi.org/10.1007/s40471-018-0172-x

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