OBJECTIVE To determine whether fetal overgrowth precedes the diagnosis of gestational diabetes mellitus (GDM) and to quantify the interrelationships among fetal overgrowth, GDM, and maternal obesity. RESEARCH DESIGN AND METHODS We conducted a prospective cohort study of unselected nulliparous women and performed ultrasonic measurement of the fetal abdominal circumference (AC) and head circumference (HC) at 20 and 28 weeks of gestational age (wkGA). Exposures were diagnosis of GDM ≥28 wkGA and maternal obesity. The risk of AC >90th and HC-to-AC ratio <10th percentile was modeled using log-binomial regression, adjusted for maternal characteristics. RESULTS Of 4,069 women, 171 (4.2%) were diagnosed with GDM at ≥28 wkGA. There was no association between fetal biometry at 20 wkGA and subsequent maternal diagnosis of GDM. However, at 28 wkGA, there was an increased risk of AC >90th percentile (adjusted relative risk 2.05 [95% CI 1.37–3.07]) and HC-to-AC ratio <10th percentile (1.97 [1.30–2.99]). Maternal obesity showed similar associations at 28 wkGA (2.04 [1.62–2.56] and 1.46 [1.12–1.90], respectively). The combination of GDM and obesity was associated with an approximately fivefold risk of AC >90th (4.52 [2.98–6.85]) and approximately threefold risk of HC-to-AC ratio <10th percentile (2.80 [1.64–4.78]) at 28 wkGA. Fetal AC >90th percentile at 28 weeks was associated with an approximately fourfold risk of being large for gestational age at birth. CONCLUSIONS Diagnosis of GDM is preceded by excessive growth of the fetal AC between 20 and 28 wkGA, and its effects on fetal growth are additive with the effects of maternal obesity.