OBJECTIVE: To evaluate differences in birth outcomes for infants born to parents in same-sex versus different-sex marriages. DESIGN: Cross-sectional study SUBJECTS: Birth certificate data from Ohio for 318,616 singleton infants born to married parents from 2016 to 2020. EXPOSURE: We estimated the associations of having a parent in a same-sex marriage (SSM) vs. a different-sex marriage (DSM). MAIN OUTCOME MEASURES: Preterm birth, low birth weight, macrosomia, small infant for gestational age, large infant for gestational age, and whether these outcomes differed by use of assisted reproductive technologies. RESULTS: A total of 1,661 singletons were born to married birthing people in SSM. Preterm birth was more common in birthing persons in SSM compared with those in DSM (11.0% vs. 7.6%), and this difference remained significant on multivariate analyses controlling for covariates. Size for gestational age was similar among infants with parents in SSM and DSM. Among those who used assisted reproductive technologies, infants born to parents in SSM had a 1.58 (95% CI: 1.15, 2.12) higher odds of macrosomia compared with those in DSM. CONCLUSION: People in SSM may face modestly increased risks for some perinatal outcomes, including preterm birth. Understanding the risks and resiliency among diverse sexual minority populations may provide important insights into the ways in which people in same-sex partnerships achieve healthy perinatal outcomes.