Objectives. Diabetes mellitus (DM) and hypothyroidism are each associated with increased rate of pregnancy complications. However, their combined morbidity during gestation is poorly studied. Therefore, the aims of this study were to determine the prevalence of the combined morbidity of DM & hypothyroidism and whether it is associated with adverse maternal and neonatal outcome. Study design. This population based retrospective cohort study included 87,213 women who had 232,293 deliveries. All deliveries were divided into the following groups: (1) hypothyroidism & DM (n = 171); (2) hypothyroidism (n = 1502); (3) DM (n = 13,324); and (4) deliveries of women with neither endocrinopathy, who served as a control group (n = 217, 296). Results. The prevalence of DM & hypothyroidism in our population was 0.17%. In comparisons to the other study groups, women with DM & hypothyroidism had higher rates of infertility (p < 0.001), preeclampsia (p < 0.001), chronic hypertension (p < 0.001), preterm birth (p < 0.001), and cesarean deliveries (p < 0.001). In Generalized Estimating Equations (GEE) model, hypothyroidism & DM was an independent risk factor for cesarean section (OR 3.46; 95% CI 2.53-4.75) and for preeclampsia (OR 1.82; 95%CI 1.16-2.84). Conclusion. The combination of DM & hypothyroidism is rare, yet it is associated with higher rate of infertility, cesarean sections, preterm deliveries, and hypertensive disorders of pregnancy than the rest of the population. This dual endocrinological combination is an independent risk factor for preeclampsia and cesarean section. These findings suggest that these patients are at risk for perinatal complications and should be followed and delivered as high risk pregnancies.