Traditionally, small for gestational age is defined as birthweight <10th percentile using sex-adjusted centile charts. However, this criterion includes constitutional variation due to maternal height, weight, ethnic group and parity. To develop customised birthweight centiles for a New Zealand population. National Women's Hospital database of births from 1993 to 2000 was used to identify eligible women with singleton pregnancies who had data available on the following: scan result for dating at gestation <24 weeks, maternal height and weight at booking, parity and ethnic origin. Multiple regression was used to determine the coefficients applicable to New Zealand. A total of 4707 pregnancies met the inclusion criteria comprising: European 1688 (36.0%), Maori 419 (8.9%) Samoan 506 (10.7%), Tongan 326 (6.9%), Chinese 751 (16.0%), Indian 214 (4.6%) and other 803 (17.1%). Mean term birthweight for an average nulliparous European woman was 3530 g. Babies of Maori and Indian ethnicity were on average 67 g and 150 g lighter, respectively, than European babies. Samoan, Tongan and Chinese babies were 84 g, 124 g and 101 g heavier, respectively. There are significant differences in birthweight between European and the other major ethnic groups in New Zealand. They relate to maternal physiological variables, for which coefficients have been derived and incorporated into freely available software that enables improved clinical assessment of fetal and neonatal weight.