BACKGROUND Obesity has become a global epidemic. Maternal overweight and obesity is now a major challenge to the treating obstetricians. We intended to find the effect of pre pregnancy body mass index (BMI) and gestational weight gain during pregnancy on maternal and neonatal outcome. METHODS 300 women with singleton pregnancy and spontaneous conception, who attended antenatal outpatient department (OPD) in our tertiary hospital, were included in this study after informed consent. The rate of gestational weight gain was calculated by subtracting the pre pregnancy weight from the final weight of the mother during last antenatal visit or during delivery. They were followed up and their maternal and perinatal outcomes such as anaemia, gestational hypertension, gestational diabetes, preterm labour, past dates, and intrauterine growth restriction, low birth weight, large for gestation, caesarean delivery and neonatal intensive care admission were studied. RESULTS The mean age of the woman in this study was 24 +/- 2 years. Of them, 13 % (n = 39) of women were underweight. 40 % (n = 120) of women were of normal BMI. 33 % (n = 99) of women were overweight. 14 % (n = 52) of women were obese. Maternal complications such as anaemia, preterm labour and intrauterine growth restriction were significantly common in underweight woman and inadequate gestational weight gain woman. Gestational diabetes, gestational hypertension, large for gestation baby and Caesarean deliveries were more in pre pregnancy obese woman and excess gestational weight gain woman. CONCLUSIONS An appropriate maternal pre pregnancy body mass index of 18.5 to 22.9 Kg / m2 at conception followed by an appropriate gestational weight gain (10 to 14 Kg) during pregnancy has a better maternal and neonatal outcome.