Publication date: Aug 01, 2025

Although numerous studies have documented no association between COVID-19 vaccination during pregnancy and maternal and fetal health outcomes, fewer studies have evaluated fetal health effects after COVID-19 vaccination around the time of conception and early pregnancy, a time when maternal exposures may affect early placentation and the subsequent risk of placenta-mediated adverse pregnancy outcomes. We used province-wide databases in Ontario to conduct a population-based cohort study including all live and stillbirths ≥20 weeks’ gestation with a last menstrual period (LMP) between April 1 and December 31, 2021. We deterministically linked birth registry data to the vaccine registry for all 80 253 eligible pregnancies; 31 209 (38. 9%) received ≥1 dose of the COVID-19 vaccine around the time of conception or first trimester. Using Cox regression, we estimated propensity score weighted hazard ratios (aHR) and 95% confidence intervals (CI) for associations between ≥1 dose of mRNA COVID-19 vaccine during the periconceptional/first trimester exposure window (28 days before the LMP to the end of first trimester) and study outcomes: hypertensive disorders (gestational hypertension, preeclampsia, eclampsia), placental abruption, preterm birth (

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