Background: Maternal body mass index (BMI) is a significant determinant of neonatal outcomes. Both underweight and overweight status before or during pregnancy may lead to adverse birth outcomes, including low birth weight (LBW), macrosomia, and neonatal complications. Methods: This prospective cross-sectional observational study was conducted at Bangabandhu Sheikh Mujib Medical University (BSMMU) and Sibchar Health Complex, Madaripur, from August 2017 to July 2018. A total of 140 pregnant women were enrolled. Maternal BMI was classified based on WHO standards, and birth outcomes including neonatal birth weight and complications were recorded. Statistical analysis was performed using SPSS version 22, and correlations between maternal BMI and neonatal outcomes were analyzed. Results: In our study, 17.9% of mothers were underweight, 46.4% had normal BMI, 25.0% were overweight, and 10.7% were obese. Low birth weight was most prevalent among underweight mothers (60.0%), whereas macrosomia occurred more frequently in overweight (11.4%) and obese mothers (26.6%). Neonatal complications, including hypoglycemia (22.9% in overweight; 40.0% in obese), respiratory distress, NICU admission, and birth injury were notably higher among neonates born to overweight and obese mothers. A statistically significant positive correlation was observed between maternal BMI and neonatal birth weight (p < 0.05). Conclusion: This study highlights a strong correlation between maternal BMI and neonatal birth weight and complications. Both low and high maternal BMI are associated with increased neonatal risks. Proper antenatal care and maternal weight management are crucial for improving perinatal outcomes.