Weight-for-length model in newborn Swedish infants

A new methodology for evaluation of weight in relation to length normalized for sex and gestational age (leanness/fatness) in the newborn infant is presented. Using standard deviation scores (SDS) for weight and length in a linear regression model, a new continuous variable called LEANSDS has been constructed with intercept = zero, regression coefficient = 0.7152, SD = 0.6988 and r2 = 0.51, irrespective of gender and gestational age within the range 34-43 weeks' gestation. The model is based on the Swedish 1986 Medical Birth Register and was tested on the 1985 year cohort. The reference material consisted of live-born, singleton infants considered "healthy". This new variable is independent of length. A chart is presented which simplifies visual evaluation. Using -2 SD as the cut-off limit, approximately one-third of light-for-dates infants are also lean, while two-thirds of the lean infants are not light for dates. Proportions of "not healthy" live-born infants and still births are given. Using another method, based on weight and length, and with some approximations needed for calculation of a comparable BW/BLSDS, the correlation was 0.9974, but this calculation is more elaborate and the comparable chart more complex. This method reveals the influence of gestational age on the relationship between weight and length, indicating a "prenatal puberty". Empirically, the logarithmic relationship found between weight and length indicates a ponderal index exponent of 2.4-2.5 instead of 3. LEANSDS, with its continuous nature, also gives the degree of deviation in each individual and in groups of infants, and can be added together and further analysed with improved sensitivity.