BACKGROUND: Antenatal ultrasound is used frequently in pregnancies complicated by hyperglycaemia; however, it is unclear which measurements have the greatest association with adverse neonatal outcomes. AIM: To assess the association between third trimester ultrasound parameters with adverse neonatal outcomes in pregnancies complicated by hyperglycaemia. METHOD: All pregnant women with gestational or type 2 diabetes who birthed in a regional hospital over 12 months were included. A composite adverse neonatal outcome was defined by one or more: admission to special care nursery, acidosis, hypoglycaemia, jaundice, shoulder dystocia, respiratory distress syndrome or 5-minute Apgar score < 5. Logistic regression was used to determine odds ratios (OR) for an adverse neonatal outcome, according to pre-specified cut points in both lower and upper percentiles of abdominal circumference (AC) and estimated fetal weight (EFW). RESULTS: Of 275 births an adverse outcome occurred in 122 (44%). Unadjusted OR (95% CI) for AC ≤30(th) was 3.2 (1.1-8.8) and >95(th) percentile was 3.1 (1.5-6.0) compared with the reference group of 31-70(th) percentile. Unadjusted OR for EFW ≤30(th) was 1.5 (0.7-3.1) and >95(th) percentile was 3.0 (1.4-6.3). After adjusting for maternal age, body mass index, diabetes type, ethnicity, gravidity, mode of delivery and gestation at birth the OR (95% CI) were as follows: AC ≤30(th) percentile, 3.7 (1.1-12.4); AC >95(th) , 2.2 (1.1-4.8); EFW ≤30(th) , 2.6 (1.1-6.1); EFW >95(th) , 2.5 (1.1-6.1). CONCLUSION: An AC and EFW up to the 30(th) percentile may pose just as great a risk to the fetus as an AC or EFW >95(th) percentile in pregnancies complicated by hyperglycaemia.