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Gestational diabetes mellitus (GDM) is associated with excessive fetal growth in later gestation. Recent data suggest accelerated growth may begin before 28 weeks' gestation when GDM is typically diagnosed. The identification of pregnancies at risk of early fetal growth would enable early intervention. We assessed the use of early pregnancy HbA1c in predicting excessive fetal growth.Women were recruited at antenatal booking from a major maternity unit in the UK. HbA1c was measured at14 weeks gestation in 1243 women at risk of GDM as defined by UK NICE risk factors of whom 1115 underwent OGTT. Women with previous GDM were excluded. Comprehensive fetal ultrasound was performed at 28 weeks' gestation alongside 75 g OGTT in 976 of these women. GDM was defined using WHO criteria. Pregnancy outcomes were extracted from the regional maternity care database.Two hundred and thirty-six women screened positive for GDM. At diagnosis, GDM pregnancies demonstrated higher adjusted fetal weight percentile than non-GDM pregnancies: (51.8 vs. 46.3, p = 0.008). This was driven by increases in the fetal abdominal circumference percentile in GDM compared with non-GDM pregnancies (55.3 vs. 46.2, p = 0.001). Early pregnancy HbA1c was higher in the GDM versus non-GDM group: (35.7 mmol/mol vs. 32.9 mmol/mol p = 0.01). A threshold for predicting excessive fetal growth was not identified in this cohort.Accelerated fetal growth is evident prior to the diagnosis of GDM. There remains a need for suitable methods of early identification of pregnancies at high risk for early accelerated fetal growth due to GDM. First-trimester HbA1c was not useful in identifying these pregnancies.WHAT IS ALREADY KNOWN?: Recent research suggests excessive growth associated with GDM may begin prior to 28 weeks' gestation, when GDM is typically tested for WHAT THIS STUDY HAS FOUND?: Pregnancies affected by GDM are already subject to accelerated fetal growth in comparison to non-GDM pregnancies by way of higher estimated fetal weight and fetal abdominal circumference Neither first-trimester HbA1c nor plasma glucose was useful predictors of these outcomes WHAT ARE THE IMPLICATIONS OF THIS STUDY?: Highlights the emergence of excessive growth prior to detection of GDM Reinforces need for suitable methods of identifying such pregnancies in earlier gestation.