Effects of gestational age at enrollment in pregnancy exposure registries.

PURPOSE: This study aims to explore the influence of gestational age at enrollment, and enrollment before or after prenatal screening, on the estimation of drug effects in pregnancy exposure registries.
METHODS: We assessed the associations between first trimester antiepileptic drug (AED) exposure and risk of spontaneous abortion and major congenital malformations in the North American AED Registry (1996-2013). We performed logistic regression analyses, conditional or unconditional on gestational age at enrollment, to estimate relative risk (RR) for first trimester AED users compared with non-users. We also compared first trimester users of valproic acid and lamotrigine. Analyses were repeated in women who enrolled before prenatal screening.
RESULTS: Enrollment occurred earlier among 7029 AED users than among 581 non-users; it was similar among AEDs. Comparing AED users with non-users, RR (95% confidence interval) of spontaneous abortion (n = 359) decreased from 5.1 (2.3-14.1) to 2.0 (0.9-5.6) after conditioning on gestational week at enrollment and to 1.9 (0.8-5.4) upon further restriction to before-screening enrollees. RR of congenital malformations (n = 216) changed from 3.1 (1.4-8.5) to 3.2 (1.4-9.0) after conditioning on gestational week at enrollment and to 2.0 (0.7-10.1) upon further restriction to before-screening enrollees. When comparing valproic acid users and lamotrigine users, the RR of congenital malformations was not substantially changed by conditioning or restricting.
CONCLUSIONS: Spontaneous abortion rates were sensitive to gestational age at enrollment. Estimates of congenital malformation risks for AED users relative to non-users were sensitive to before/after-screening enrollment. This difference was not apparent between active drugs, likely due to similar gestational age at enrollment.