OBJECTIVE: To evaluate the effects of extreme obesity (pre-pregnancy BMI ≥ 50.0 kg/m2) in pregnancy on maternal and perinatal outcomes. METHODS: We conducted a population-based cohort study using the Newfoundland and Labrador Perinatal Database to compare obstetric outcomes in women with extreme obesity and those with a normal BMI (pre-pregnancy BMI 18.50 to 24.99 kg/m2). We included women with singleton gestations who gave birth between January 1, 2002, and December 31, 2011. Maternal outcomes of interest included gestational hypertension, gestational diabetes, Caesarean section, shoulder dystocia, length of hospital stay, maternal ICU admission, postpartum hemorrhage, and death. Perinatal outcomes included birth weight, preterm birth, Apgar score, neonatal metabolic abnormality, NICU admission, stillbirth, and neonatal death. A composite morbidity outcome was developed including at least one of Caesarean section, gestational hypertension, birth weight ≥ 4000 g, birth weight < 2500 g, or NICU admission. Univariate analyses and multivariate logistic regression analyses (controlling for maternal age, parity, smoking, partner status, and gestational age) were performed, and adjusted odds ratios (aORs) and 95% confidence intervals were calculated. RESULTS: A total of 5788 women were included in the study: 71 with extreme obesity and 5717 with a normal BMI. Extremely obese women were more likely to have gestational hypertension (19.7% vs. 4.8%) (aOR 1.56; 95% CI 1.33 to 1.82), gestational diabetes (21.1% vs. 1.5%) (aOR 2.04; 95% CI 1.74 to 2.38), shoulder dystocia (7.1% vs. 1.4%) (aOR 1.51; 95% CI 1.05 to 2.19), Caesarean section (60.6% vs. 25.0%) (aOR 1.46; 95% CI 1.29 to 1.65), length of hospital stay more than five days (excluding Caesarean section) (14.3% vs. 4.7%) (aOR 1.42; 95% CI 1.07 to 1.89), birth weight ≥ 4000 g (38.0% vs. 11.9%) (aOR 1.58; 95% CI 1.38 to 1.80), birth weight ≥ 4500 g (16.9% vs. 2.1%) (aOR 1.87; 95% CI 1.57 to 2.23), neonatal metabolic abnormality (8.5% vs. 2.0%) (aOR 1.50; 95% CI 1.20 to 1.86), NICU admission (16.9% vs. 7.8%) (aOR 1.28; 95% CI 1.07 to 1.52), stillbirth (1.4% vs. 0.2%) (aOR 1.68; 95% CI 1.00 to 2.82) and composite adverse outcome (81.7% vs. 41.5%) (aOR 1.57; 95% CI 1.35 to 1.83). CONCLUSION: Women with extreme obesity have increased risks of a variety of adverse maternal and perinatal outcomes. As approximately 6 per 1000 women giving birth in our population have extreme obesity, it is important to address these risks pre-conceptually and encourage a healthier BMI before pregnancy.