Objective To determine the association between fetal biometry in the first or early second trimester and severe macrosomia at delivery. Methods This case-control study included 30 term severely macrosomic neonates; 90 appropriate-for-gestational age (AGA) neonates served as controls. All pregnancies underwent nuchal translucency (NT) screening at 11 - 14 weeks' gestation. Pregnancies were dated by accurate last menstrual period consistent with Crown-rump length (CRL) measurements at the time of screening, early pregnancy, CRL of date of fertilization. The association between birth weight and the difference between The measured and the expected CRL at the time of NT screening was analyzed. Results The difference between measured and expected CRL, expressed both in mm and in days of gestation, was statistically greater in the severely macrosomic. C neonates compared with controls (mean, 6.66 +/- 4.78 mm vs. 1.17 +/- 4.6 mm, P < 0.0001 and 3 +/- 2.2 days vs. 0.5 +/- 2.3 days, P < 0.0001, respectively). Futhermore, there were significant correlations between the extent of macrosomia and the discrepancy between expected and measured fetal size at the time of NT Screening (r = 0.47, P < 0.01 and r = 0.48, P < 0.01, respectively). Conclusion Severe macrosomia apparently manifests as early as 11 - 14 weeks' gestation. Copyright (c) 2008 ISUOG. Published by John Wiley & Sons, Ltd.