Objective To examine if a “dose–response” relation exists between different classes of pre-gravid obesity and selected perinatal outcomes. Methods We evaluated 16,566 obese mothers, including 12,064 (72.8%), 3410 (20.6%), and 1092 (6.6%) with obesity class I, II, and III, respectively. We compared maternal age, primiparity, gestational age at birth, birth weight, GDM, hypertensive disorders, and the incidence of cesarean sections. Results There was a significantly increased incidence (from class I to class III) for GDM (8.5–14.4%), chronic hypertension (2.8–9.0%), gestational hypertension (6.7–14.2%), and for preeclampsia (5.3–9.3%). No such relationship existed for birth weight and gestational duration. Conclusion Classes of obesity during pregnancy exhibit a “dose–response” relationship with maternal morbidity, but no such relationship was found with pregnancy duration and birth weight.