<b><i>Background:</i></b> The risk factors determining the frequency of intrauterine transfusions (IUTs) for severely affected red blood cell alloimmunized singleton pregnancies are not well known. <b><i>Objective:</i></b> To assess factors associated with IUT frequency and adverse pregnancy outcomes in transfused pregnancies<i>.</i><b><i>Methods:</i></b> Retrospective cohort analysis of 246 consecutive cases between 1991 and 2014. Time-to-event survival analysis for repeated events was used to evaluate risk of subsequent IUT. Multivariable logistic regression assessed odds of a composite adverse pregnancy outcome (intrauterine fetal death, termination of pregnancy, neonatal death, preterm birth <34 weeks’ gestation). <b><i>Results:</i></b> Full information was available on<i></i>232 cases (94.3%) and 716 IUTs. Fetal hydrops was associated with increased frequency (hazard ratio [HR] 1.29 [95% CIs 1.15–1.47, <i>p</i> < 0.001]) while higher fetal hemoglobin (Hb) pre-IUT (HR) 0.99 (95% CI 0.99–1.00, <i>p</i> = 0.021) and post-IUT (HR 0.99 [95% CI 0.99–1.00] <i>p</i> = 0.042), and higher transfused blood volume (HR 0.98 [95% CI 0.97–0.99] <i>p</i> < 0.001) were associated with reduced IUT frequency. Adverse pregnancy outcomes were more likely with lower gestational age (GA) at initial IUT. Antibody type was not associated with IUT frequency or adverse pregnancy outcomes. <b><i>Conclusions:</i></b> Hydrops is associated with increased IUT frequency while lower GA at initial IUT is associated with higher adverse pregnancy outcomes in alloimmunized pregnancies.<i></i>Higher transfused blood volumes, pre- and post-IUT Hb are associated with lower IUT frequency.