Objectives Abnormal body mass index (BMI) during pregnancy, a growing public health concern, increases maternal and neonatal complications. This study aimed to investigate the impact of abnormal BMI on perinatal outcomes compared to normal BMI. Methods A total of 14,624 women having singleton births were categorized as underweight (BMI<18.5 kg/m 2 ), overweight (25.0–29.9 kg/m 2 ), obesity class I (30.0–34.9 kg/m 2 ), obesity class II (35.0–39.9 kg/m 2 ), and obesity class III (≥40.0 kg/m 2 ) and compared to those with normal BMI (18.5–24.9 kg/m 2 ). Outcomes included gestational diabetes (GDM), gestational hypertension (GHT), postpartum haemorrhage (PPH), cesarean delivery (CD), preterm birth (PTB), low birth weight (LBW), congenital anomalies and neonatal intensive care unit admission. Results Women with increasing BMI had increasingly higher odds of developing specific adverse outcomes, the highest being in the class III obesity group (GDM-aOR 2.71, 95 % CI 2.25–3.27, p<0.001, GHT-aOR 5.32 95 % CI 3.49–8.11, p<0.001, CD-aOR 2.33 95 % CI 1.85–2.94, p<0.001, PPH-aOR 1.77 95 % CI 1.35–2.33, p<0.001). On the other hand, being underweight during pregnancy was associated with increased odds of PTB (aOR 2.09, 95 % CI 1.37–3.20, p=0.001), LBW (OR 1.88, 95 % CI 1.27–2.79, p=0.002) and congenital anomalies (aOR 2.52 95 % CI 1.12–5.64, p=0.025). Majority in the underweight category gained less than expected gestational weight gain during the pregnancy. Conclusions The findings of this study have important implications for the clinical management of pregnant women with abnormal BMI. Interventions to improve maternal and neonatal outcomes must focus on enhancing pre-pregnancy BMI and maintaining adequate gestational weight gain.