<p><strong>BACKGROUND: </strong>Intrapartum antibiotic prophylaxis (IAP) reduce a newborn's risk of group B streptococcal infection (GBS) but may lead to an increased childhood body mass index (BMI).</p>

<p><strong>METHODS: </strong>Retrospective cohort study of infants (n=223,431) born 2007-2015 in an integrated healthcare system. For vaginal delivery, we compared children exposed to GBS-IAP and to any other type or duration of intrapartum antibiotics to no antibiotic exposure. For Cesarean delivery, we compared children exposed to GBS-IAP to those exposed to all other intrapartum antibiotics, including surgical prophylaxis. BMI over 5 years was compared using non-linear multivariate models with B-spline functions, stratified by delivery mode and adjusted for demographics, maternal factors, breastfeeding and childhood antibiotic exposure.</p>

<p><strong>RESULTS: </strong>In vaginal deliveries, GBS-IAP was associated with higher BMI from 0.5 to 5.0 years of age compared to no antibiotics (P&lt;0.0001 for all time points, Δ BMI at age 5 years 0.12&nbsp;kg/m 2, 95% CI 0.07 to 0.16&nbsp;kg/m 2). Other antibiotics were associated with higher BMI from 0.3 to 5.0 years of age. In Cesarean deliveries, GBS-IAP was associated with increased BMI from 0.7 years to 5.0 years of age (P&lt;0.05 for 0.7-0.8 years, P&lt;0.0001 for all other time points) compared to other antibiotics (Δ BMI at age 5 years 0.24&nbsp;kg/m 2, 95% CI 0.14 to 0.34&nbsp;kg/m 2). Breastfeeding did not modify these associations.</p>

<p><strong>CONCLUSION: </strong>GBS-IAP was associated with a small but sustained increase in BMI starting at very early age. This association highlights the need to better understand the effects of perinatal antibiotic exposure on childhood health.</p>