Background: The increasing prevalence of obesity and sever decrease in maternal weight In young women ( underweight) are major health problems. These trends have a major effects on pregnancy outcomes in these women which have been documented by several researches. This paper examine the effect of increasing Body Mass index (BMI) on pregnancy outcomes in nulliparous women delivering singleton babies.Methods: This is a prospective study , including 100 pregnant women, all nulliparous women delivering singleton babies during year 2010, women were categorized into two groups – normal ( BMI 20- 25 Kg/m2) and obese ( BMI 30-40 kg/m2) . Obstetric and perinatal outcomes were compared.Results: This study showed increased incidence of macrosomic babies (6,3%) in obese women than in normal women , increased rates of shoulder dystocia (3,8%) rate of admission to neonatal intensive care unites (20%). And primary post partum hemorrhage (2%) than in normal weight women . No statistical significance diffrence was found concerning IUGR, delayed wound healing, wound infection .Conclusion: Increasing BMI is associated with increased incidence of failed progress of labour, labour induction ,instrumental delivery , macrosomic babies and shoulder dystocia.