Background: Faster-acting insulin aspart (faster aspart) is considered safe for use during pregnancy and breastfeeding but has not been evaluated in this population. We aimed to evaluate the effect of faster aspart versus insulin aspart on fetal growth, in women with type 1 or type 2 diabetes during pregnancy and post-delivery. Methods: This open-label, single-centre, superiority trial was conducted at Rigshospitalet, Copenhagen, Denmark. Participants aged 18 years or older with type 1 or type 2 diabetes were stratified by diabetes type and insulin treatment modality (multiple daily injections or insulin pump), randomly assigned 1:1 to faster aspart or insulin aspart, from 8 weeks and 0 days (8⁺⁰) of gestation to 13⁺⁶ weeks of gestation, and followed up until 3 months post-delivery. Primary outcome was infant birthweight SD score. Secondary outcomes included HbA_1c as well as maternal and fetal outcomes in all participants during the trial. This trial is registered with ClinicalTrials.gov, NCT03770767. Findings: Between Nov 11, 2019 and May 10, 2022, 109 participants were included in the faster aspart group and 107 in the insulin aspart group. Primary outcome data were available in 203 (94%) of 216 participants, and no participants discontinued treatment during the trial. Mean birthweight SD score was 1·0 (SD 1·4) in the faster aspart group versus 1·2 (1·3) in the insulin aspart group; estimated treatment difference –0·22 [–0·58 to 0·14]; p=0·23. At 33 weeks of gestation, mean HbA_1c was 42 mmol/mol (SD 6 mmol/mol; 6·0% [SD 0·9%]) versus 43 mmol/mol (SD 7 mmol/mol; 6·1% [SD 1·2%]); estimated treatment difference –1·01 (–2·86 to 0·83), p=0·28. No additional safety issues were observed with faster aspart compared with insulin aspart. Interpretation: Treatment with faster aspart resulted in similar fetal growth and HbA_1c, relative to insulin aspart, in women with type 1 or type 2 diabetes. Faster aspart can be used in women with type 1 or type 2 diabetes during pregnancy and post-delivery with no additional safety issues. Funding: Novo Nordisk. Translation: For the Danish translation of the abstract see Supplementary Materials section.