SummaryBackground/ObjectivesCurrent research suggests an association between antibiotic use in early life and later obesity. Less is known about prenatal antibiotic exposure and foetal growth. We investigated the association between prenatal antibiotic exposure and birth weight.MethodsData from the Danish National Birth Cohort were linked to the Danish National Medical Birth Registry. Exposure was self‐reported antibiotic use in pregnancy. Outcome was registered birth weight. Multivariable linear regression models were adjusted for confounders defined a priori.ResultsA total of 63 300 mother–child dyads from 1996 to 2002 were included. Overall, prenatal antibiotic exposure was not associated with birth weight (−8.90 g, 95%CI: −19.5– +1.64 g, p = 0.10). Findings were similar for those born term and preterm. Antibiotic exposure in second to third trimester, compared to no exposure, was associated with lower birth weight (−12.6 g, 95%CI: −24.1 to −1.1 g, p = 0.03). In sex‐stratified analyses, there were no observed associations between antibiotics and birth weight. With further stratifications, prenatal antibiotic exposure and birth weight were associated in boys who were preterm (+91.0 g, 95%CI: +6.8 g– +175.2 g, p = 0.03) but not among girls who were preterm (−44.0 g, 95%CI: −128.1 to +40.0 g, p = 0.30).ConclusionsPrenatal antibiotic exposure is not consistently associated with birth weight.