Background: Our aim was to identify and compare modifiable risk factors associated with adverse pregnancy outcomes in women with type 1 and type 2 diabetes and to identify effective maternity clinics. Methods:We included 17,375 pregnancies in 15,290 women with diabetes in a populationbased cohort study across 172 maternity clinics in England, Wales and the Isle of Man.Obstetric complications (preterm delivery, large birthweight) and adverse pregnancy outcomes (congenital anomaly, stillbirth, neonatal death) were obtained for pregnancies completed between 01 January 2014 and 31 December 2018. We assessed associations between modifiable (glycaemia, obesity, clinic) and non-modifiable risk factors (age, deprivation, ethnicity) with pregnancy outcomes.Results: Of 17,375 pregnancies, 8,690 (50.0%) were in women with type 1 and 8,685 (50.0%) in women with type 2 diabetes. The rates of preterm delivery (42.5% type 1, 23.4% type 2), and large birthweight (52.2% type 1, 26.2% type 2) were higher in type 1 diabetes (p<0.001).The prevalence of congenital anomaly (44.8/1000 type 1, 40.5/1000 type 2; p=0.175), and stillbirth (10.4/1000 type 1, 13.5/1000 type 2; p=0.072) did not differ but neonatal death rates (7.4/1000 type 1, 11.2/1000 type 2; p=0.013) were higher in type 2 diabetes. Independent risk factors for perinatal death were third trimester HbA1c > 48mmol/mol (OR 3.06, 95% CI 2.16 to 4.33), living in the highest deprivation quintile (OR 2.29 95% CI 1.16 to 4.52) and having type 2 diabetes (OR 1.65 95% CI 1.18 to 2.31). Variations in glycaemia and large birthweight were associated with maternal characteristics (diabetes duration, deprivation, BMI) without substantial differences between clinics.Interpretation: Our data highlight persistent adverse pregnancy outcomes in type 1 and type 2 diabetes. Maternal glycaemia and obesity are the key modifiable risk factors. No clinics were achieving appreciably better outcomes, suggesting that healthcare system changes are needed