The objective of this study was to create and validate new intrauterine weight, length, and head circumference growth curves using a contemporary, large, racially diverse US sample and compare with the Lubchenco curves. Data on 391 681 infants (Pediatrix Medical Group) aged 22 to 42 weeks at birth from 248 hospitals within 33 US states (1998-2006) for birth weight, length, head circumference, estimated gestational age, gender, and race were used. Separate subsamples were used to create and validate curves. Smoothed percentile curves (3rd to 97th) were created by the Lambda Mu Sigma (LMS) method. The validation sample was used to confirm representativeness of the curves. The new curves were compared with the Lubchenco curves. Final sample included 257 855 singleton infants (57.2% male) who survived to discharge. Gender-specific weight-, length-, and head circumference-for-age curves were created (n = 130 111) and successfully validated (n = 127 744). Small-for-gestational age and large-for-gestational age classifications using the Lubchenco curves differed significantly from the new curves for each gestational age (all P < .0001). The Lubchenco curves underestimated the percentage of infants who were small-for-gestational-age except for younger girls (< or =36 weeks), for whom it was more likely to be overestimated; underestimated percentage of infants (< or =36 weeks) who were large-for-gestational-age; and overestimated percentage of infants (>36 weeks) who were large-for-gestational-age. The Lubchenco curves may not represent the current US population. The new intrauterine growth curves created and validated in this study, based on a contemporary, large, racially diverse US sample, provide clinicians with an updated tool for growth assessment in US NICUs. Research into the ability of the new definitions of small-for-gestational-age and large-for-gestational-age to identify high-risk infants in terms of short-term and long-term health outcomes is needed.