The Relationship between COVID-19 Vaccination Status in Pregnancy and Birth Weight

BACKGROUND: Despite findings that maternal COVID-19 infection in pregnancy is associated with low birth weight (LBW; weight < 2,500 grams), previous studies demonstrate no difference in LBW risk between COVID-19 vaccinated and unvaccinated pregnant persons. Few studies, however, have examined the association of unvaccinated, incomplete, and complete vaccination on LBW, and they have been limited by small sample sizes and lack of adjustment for covariates. OBJECTIVE: We sought to address key limitations of prior work and evaluate this association between unvaccinated, incomplete, and complete COVID-19 vaccination status in pregnancy and LBW. We predicted a protective association of vaccination on LBW that varies by number of doses received. STUDY DESIGN: We performed a population-based retrospective study using the Vizient® clinical database, which included data from 192 hospitals in the United States. Our sample included pregnant persons who delivered between January 2021 and April 2022 at hospitals that reported maternal vaccination data and birth weight at delivery. Pregnant persons were categorized into three groups: unvaccinated; incompletely vaccinated (one dose of Pfizer or Moderna); or completely vaccinated (one dose of Johnson & Johnson or ≥ two doses of Moderna or Pfizer). Demographics and outcomes were analyzed using standard statistical tests. We performed multivariable logistic regression to account for potential confounders between vaccination status and LBW in the original cohort. Propensity score matching was used to reduce bias related to likelihood of vaccination, and the multivariable logistic regression model was then applied to the propensity score matched cohort. Stratification analysis was performed for gestational age and race/ethnicity. RESULTS: Of the 377,995 participants, 31,155 (8.2%) had LBW, and these participants were more likely to be unvaccinated than those without LBW (98.8% versus 98.5%, p < 0.001). Incompletely vaccinated pregnant persons were 13% less likely to have LBW neonates compared to unvaccinated persons (odds ratio 0.87, 95% confidence interval 0.73-1.04), and completely vaccinated persons were 21% less likely to have LBW neonates (odds ratio 0.79, 95% confidence interval 0.79-0.89). After controlling for maternal age, race/ethnicity, hypertension, diabetes, lupus, tobacco use, multifetal gestation, obesity, use of assisted reproductive technology, and maternal/neonatal COVID-19 infections in the original cohort, these associations remained significant for only complete vaccination (adjusted odds ratio 0.80, 95% confidence interval 0.70-0.91) and not incomplete vaccination (adjusted odds ratio 0.87, 95% confidence interval 0.71-1.04). In the propensity score matched cohort, pregnant persons who were completely vaccinated against COVID-19 were 22% less likely to have LBW neonates compared to unvaccinated and incompletely vaccinated individuals (aOR 0.78, 95% CI 0.76-0.79). CONCLUSION: Pregnant persons who were completely vaccinated against COVID-19 were less likely to have LBW neonates compared to unvaccinated and incompletely vaccinated individuals. This novel association was observed among a large population after adjusting for confounders of LBW and factors influencing the likelihood of receiving the COVID-19 vaccine.