OBJECTIVE: To study perinatal outcomes in singletons born after intrauterine insemination (IUI) compared with children born after in vitro fertilization (IVF), intracytoplasmic sperm injection, and spontaneous conception (SC), and to assess predictors of poor outcome in singletons born after IUI, exploring the effect of ovarian stimulation. DESIGN: National cohort study, 2007-2012. SETTING: Danish national registries. PATIENT(S): Four thousand two hundred twenty-eight singletons born after insemination with partner semen (IUI-H) and 1,881 singletons born after insemination with donor semen. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Preterm birth (PTB), low birth weight (LBW), small for gestational age (SGA). RESULT(S): Children born after IUI-H had higher risks of PTB, LBW, and SGA vs. SC singletons (adjusted odds ratios [aOR] 1.3; 95% confidence interval (CI), 1.1-1.5; 1.4; 95% CI, 1.2-1.7; and 1.4; 95% CI, 1.2-1.6), respectively. Compared with IVF, risk of SGA was similar, but risks of PTB and LBW were lower (aOR 0.6; 95% CI, 0.5-0.8; and 0.8; 95% CI, 0.6-0.9). Compared with intracytoplasmic sperm injection, no differences were found. For children born after IUI with donor semen, results were similar to those for IUI-H. Risks of LBW and SGA were higher in IUI singletons born after ovarian stimulation with clomiphene citrate, compared with natural-cycle IUI (aOR 1.5; 95% CI, 1.1-2.1 and 1.6; 95% CI, 1.1-2.4). Treatment with follicle-stimulating hormone vs. natural-cycle IUI did not seem to affect perinatal outcomes. CONCLUSION(S): Singletons born after IUI had higher risk of adverse perinatal outcomes compared with SC children, similar to ICSI, but favorable outcomes compared with IVF. Stimulation with clomiphene citrate was associated with higher risk of SGA compared with natural-cycle IUI, but follicle-stimulating hormone treatment did not seem to be associated with adverse outcomes.