The increasing prevalence of obesity is presenting a critical challenge to healthcare services. We examined the effect of Body Mass Index in early pregnancy on adverse pregnancy outcome. We performed a population register-based cohort study using data from the North Western Perinatal survey (N = 99,403 babies born during 2004-2006), based at The University of Manchester, UK. The main outcome measures were Caesarean section delivery, preterm birth, neonatal death, stillbirth, Macrosomia, small for gestational age and large for gestational age. The risk of preterm birth was reduced by almost 10% in overweight (RR = 0.89, [95% CI: 0.83, 0.95]) and obese women (RR = 0.90, [95% CI: 0.84, 0.97]) and was increased in underweight women (RR = 1.33, [95% CI: 1.16, 1.53]). Overweight (RR = 1.17, [95% CI: 1.09, 1.25]), obese (RR = 1.35, [95% CI: 1.25, 1.45]) and morbidly obese (RR = 1.24, [95% CI: 1.02, 1.52]) women had an elevated risk of post-term birth compared to normal women. The risk of fetal macrosomia and operative delivery increased with BMI such that morbidly obese women were at greatest risk of both (RR of macrosomia = 4.78 [95% CI: 3.86, 5.92] and RR of Caesarean section = 1.66 [95% CI: 1.61, 1.71] and a RR of emergency Caesarean section = 1.59 [95% CI: 1.45, 1.75]). Excessive leanness and obesity are associated with different adverse pregnancy outcomes with major maternal and fetal complications. Overweight and obese women have a higher risk of macrosomia and Caesarean delivery and lower risk of preterm delivery. The mechanism underlying this association is unclear and is worthy of further investigation.