Abstract: Excessive fetal growth due to GDM occurs despite screening and intervention. US & UK guidelines recommend screening for GDM between 24 and 28 weeks of gestational age (wGA). We sought to determine whether excessive fetal growth preceded clinical diagnosis of GDM. We performed a prospective cohort study of 4,512 nulliparous women with a singleton pregnancy in Cambridge (UK), 2008-2013. All women had 2 stage screening for GDM: a 1 hour, non-fasting 50g glucose challenge test (GCT) at ∼28wGA, followed by a 2 hour, fasting 75g oral glucose tolerance test if the GCT was positive. All women had ultrasonic measurement of the fetal abdominal circumference (AC) and head circumference (HC) at 20 and 28wGA. Differences were quantified as standard deviations (SD) from the mean at the given GA, adjusted for maternal characteristics (age, body mass index, height and ethnicity). Among 4,002 eligible women, 167 (4.2%) were diagnosed with GDM at ≥28wGA. At 20wGA, there were no significant difference in any fetal biometric measurements in pregnancies subsequently diagnosed with GDM. However, at 28wGA, these fetuses had increased AC (0.29 SD greater, 95% CI 0.13 to 0.44), reduced HC:AC ratio (0.16 SD smaller, 95% CI 0.00 to 0.32), and increased AC growth velocity from 20 to 28wGA (0.20 SD greater, 95% CI 0.04 to 0.37). Maternal obesity was associated with a similar increase in the AC at 28wGA and the effects of obesity and GDM were additive (Figure): the combination was associated with a 0.60 SD greater AC and a 0.40 SD smaller HC:AC ratio. The associations with GDM were very similar when (i) the analyses were confined to women who had normal postpartum glucose tolerance, and (ii) using either WHO or IADPSG diagnostic criteria for GDM. Diagnosis of GDM ≥28wGA is preceded by excessive growth of the fetal AC, and this is most likely due to hyperglycemia caused by GDM prior to 28wGA. Screening for GDM at 24wGA may result in better prevention of fetal macrosomia.