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The objective of this study was to test the hypothesis that fetuses destined to deliver preterm do not reach their individual growth potential.In a case control design, data on 44 preterm deliveries ator =34 weeks were compared with data on a control group of next consecutive term deliveries. Criteria for inclusion were dating by ultrasonography at20 weeks and no medical or obstetric complications. For each fetus, GROW v.2 software was used to generate an individual optimal growth curve and to calculate the percentile of achieved growth potential for birth weight based on 6 independent factors (maternal weight, height, parity, ethnicity, fetal sex, and gestational age) identified as determining fetal weight from multivariate logistic regression analysis of 40,000 uncomplicated term pregnancies. Birth weight percentiles based on standard norms were also calculated for each fetus.The number of fetuses with birth weight below the 5th, 10th, and 25th percentile of their growth potential was significantly higher in the preterm group (10, 13, and 18) compared with that in the control group (2, 2, and 6; P.008, P.001, and P.008, respectively). There were no significant differences in variables defining growth potential between the case and control groups. The number of fetuses below the 5th and 10th percentile based on standard birth weight norms was not significantly different between preterm and term pregnancies (3 vs 1 and 5 vs 2; P =.37 and P =.27). Among preterm deliveries, those preceded by preterm premature rupture of the membranes had significantly fewer fetuses75th percentile of their growth potential (2 vs 8; P =.025). Fetuses with lower gestational ages at preterm delivery achieved lower median percentiles of their growth potential.A significant proportion of fetuses destined to deliver preterm do not reach their individual growth potential compared with those delivered at term. This finding challenges our concept of preterm delivery and management strategy aimed at tocolysis.