ObjectiveTo examine birth outcomes between children conceived with in vitro fertilization (IVF) or intrauterine insemination (IUI) and sibling births from unassisted conceptions.DesignRetrospective sibling cohort.SubjectsLive born children conceived via IVF, with or without intracytoplasmic sperm injection, or IUI at the Utah Center for Reproductive Medicine, 1999-2018, and sibling births from unassisted conceptions (born 1985-2018). The main analysis included singleton births (460 IVF, 666 IUI, and 1,579 unassisted siblings).ExposureIn vitro fertilization, with or without intracytoplasmic sperm injection, or IUI.Main outcome measuresPreterm birth, low birth weight, small for gestational age, large for gestational age (LGA), and major congenital anomalies.ResultsCompared with unassisted siblings, singleton children conceived via IVF had gestational ages shorter by nearly half a week (95% confidence interval [CI], -0.6 to -0.3), birth weights of 72.1 g lower (95% CI, -118.8 to -25.4), and higher proportions of preterm birth (IVF, 11.1%; IUI, 8.7%; unassisted siblings, 6.8%), LGA (IVF, 9.1%; IUI, 4.5%; unassisted siblings, 5.9%), and major congenital anomalies (IVF, 3.7%; IUI, 2.0%; unassisted siblings, 1.4%). Models adjusted for maternal age, infant sex, infant birth year, previous pregnancy, and birth order showed that children conceived via IVF were more likely to be preterm (adjusted risk ratio [aRR], 1.6; 95% CI, 1.2-2.2; absolute difference, 4.3%) and LGA (aRR, 1.8; 95% CI, 1.2-2.5; absolute difference, 3.2%). Children conceived via IVF had a higher risk of major congenital anomalies than unassisted siblings adjusted for maternal age, infant sex, and birth order (aRR, 1.9; 95% CI, 1.0-3.8; absolute difference, 2.3%). Children conceived via IUI had birth weights 55.8 g lower (95% CI, -95.6 to -15.9) than unassisted siblings.ConclusionWe observed an increased risk of preterm birth, low birth weight, LGA, and major congenital anomalies among singleton children conceived with IVF compared with that among unassisted siblings; however, absolute differences remain small. For children conceived via IUI, lower birth weights were observed. These results suggest that treatment-related factors in addition to underlying subfertility may contribute to adverse birth outcomes.