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Title: Estimating gestational age-specific exposure effects during pregnancy with observational data: A target trials approach Authors:  Mireille Schnitzer - Universite de Montreal (Canada) [presenting]
Steve Ferreira Guerra - McGill University (Canada)
Cristina Longo - Universite de Montreal (Canada)
Lucie Blais - Universite de Montreal (Canada)
Robert Platt - McGill University (Canada)
Abstract: Many studies seek to evaluate the effects of potentially harmful pregnancy exposures during specific gestational periods. We consider an observational pregnancy cohort where women can initiate medication usage or become exposed to a drug at various times during their pregnancy. An important statistical challenge involves defining and estimating exposure effects when pregnancy loss or delivery can occur over time. Without proper consideration, the results of standard analyses may be vulnerable to selection bias, immortal time-bias, and time-dependent confounding. We apply the target trials framework of Hernan and Robins in order to define effects based on the counterfactual approach often used in causal inference. This effect is defined relative to a hypothetical randomized trial of timed pregnancy exposures where delivery may precede (and thus interrupt) exposure initiation. We demonstrate tailored implementations of inverse probability weighting (IPW), G-Computation, and Targeted Maximum Likelihood Estimation (TMLE) to estimate the effects of interest. We then apply our proposed methods to a pharmacoepidemiology study to evaluate the potentially time-dependent effect of exposure to inhaled corticosteroids on birth weight in pregnant women with mild asthma.