Maternal characteristics and expected birth weight.

Fetal growth charts currently used aggregate birth weights of infants with various natural histories from 1931 until 1967. In order to modernize these charts, avoiding deviation from the natural history of fetal development, we report data from infants born after spontaneous onset of labour in 'normal' pregnancy from a gestational age of 267 to 295 days between 1972 and 1982 (n = 14,113). The relationship between birth weight and gestational age in days was studied by multiple regression analysis, containing dummy variables for parity and gender. The estimated proportion of the variance in the model, attributed to these characteristics, was 15%. This could be improved to 22% by supplementing the model with maternal characteristics such as age, height, mid-pregnancy weight and ethnic origin. According to this extended model, in the Dutch section of the population 511 (4.6%) babies had a birth weight below the 5th percentile, whereas 412 (3.7%) babies would be labeled as such according to the conventional birth weight tables. Moreover, 93 babies would be wrongly considered too small, corresponding with a sensitivity of 62.4%, and 192 babies would be wrongly considered normal, corresponding with a specificity of 99.3%. Integration of the four currently used tables into one, and adjustment for easily available maternal characteristics, could substantially improve classification methods.