Objectives: This study aimed to evaluate the risk of congenital malformation among pregnant women exposed to the mRNA COVID-19 vaccines during the first trimester of pregnancy, which is a developmental period where the foetus is at risk of teratogenicity.

Methods: Pregnant women were prospectively enrolled from March 2021 to March 2022, at the time of COVID-19 vaccination. Pregnant women exposed to at least one dose of mRNA COVID-19 vaccine from conception to 11 weeks of gestations and 6 days were compared with pregnant women exposed to the vaccine from 12 weeks to the end of pregnancy. The primary outcome was a confirmed congenital malformation at birth.

Results: A total of 1450 pregnant women were enrolled including 124 in the first trimester and 1326 in the second and third trimester. The overall proportion of congenital malformation was 0.81% (n = 1/124; 95% CI: 0.02-4.41) and 0.83% (n = 11/1326; 95% CI: 0.41-1.48) among pregnant exposed to the COVID-19 vaccine during the first and second/third trimester, respectively. First trimester exposure was not associated with a higher risk of congenital malformation with a relative risk of 0.89 (95% CI: 0.12-6.80) with no significant changes after adjustment through exploratory analysis.

Conclusions: Pregnant women exposed to mRNA COVID-19 vaccine before 12 weeks of gestation did not have an increased risk of congenital malformation compared with women exposed outside the teratogenic window. Because vaccination is safe and effective, emphasis must be placed on promoting vaccination during pregnancy.