WEIGHT-FOR-LENGTH MODEL IN NEWBORN SWEDISH INFANTS

A new methodology for evaluation of weight in relation to length norma lized for sex and gestational age (leanness/fatness) in the newborn in fant is presented. Using standard deviation scores (SDS) for weight an d length in a linear regression model, a new continuous variable calle d LEANSDS has been constructed with intercept = zero, regression coeff icient = 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 th e 1985 year cohort. The reference material consisted of live-born, sin gleton infants considered ''healthy''. This new variable is independen t of length. A chart is presented which simplifies visual evaluation. Using - 2 SD as the cut-off limit, approximately one-third of light-fo r-dates infants are also lean, while two-thirds of the lean infants ar e not light for dates. Proportions of ''not healthy'' live-born infant s and still births are given. Using another method, based on weight an d length, and with some approximations needed for calculation of a com parable BW/BLSDS, the correlation was 0.9974, but this calculation is more elaborate and the comparable chart more complex. This method reve als the influence of gestational age on the relationship between weigh t and length, indicating a ''prenatal puberty''. Empirically, the loga rithmic relationship found between weight and length indicates a ponde ral index exponent of 2.4-2.5 instead of 3. LEANSDS, with its continuo us nature, also gives the degree of deviation in each individual and i n groups of infants, and can be added together and further analysed wi th improved sensitivity.