We aimed to identify potentially modifiable risk factors and causes for preterm delivery in women with type 1 or type 2 (pre-existing) diabetes. A secondary analysis of a prospective cohort study of 203 women with pre-existing diabetes (117 type 1 and 86 type 2 diabetes) was performed. Consecutive singleton pregnancies were included at the first antenatal visit between September 2015 and February 2018. In total, 27% (n = 55) of the 203 women delivered preterm at median 36 + 0 weeks. When stratified by diabetes type, 33% of women with type 1 diabetes delivered preterm compared with 20% in women with type 2 diabetes (p = 0.04). Women delivering preterm were characterised by a higher prevalence of pre-existing kidney involvement (microalbuminuria or diabetic nephropathy) (16% vs 3%, p = 0.002), preeclampsia (26% vs 5%, p