[Construction of model for calculating and recording neonatal weight percentiles]

To construct a model for calculating optimal foetal and neonatal weight curves with a method that allows automatic calculation of the percentile and sequential recording of results.

A model was constructed for calculating optimal weight and the corresponding percentiles for gestational age and sex from a sample of 23,578 newborns, after excluding cases with diseases. Birth weight was modelled using stepwise multiple regression analysis. Newborns were classified as small or large for gestational age (SGA or LGA) using the proposed model. The resulting classification was compared with those derived from other models designed for Spanish children.

Optimal weight model: 3,311.062+68.074 *sex+143.267 *GE40 -13.481 *GE40(2) - 0.797 *GE40(3)+sex* (5.528 *GE40 - 0.674 *GE40(2) - 0.064 *GE40(3)). (GE, gestational age). Weight percentiles were obtained from standardized data using the coefficient of variation of the optimal weight. The degree of agreement between our model classification and those of the Carrascosa model and Ramos model, with empirical and smooth percentiles, was "almost perfect" (κ=0.866, κ=0.872, and κ=0.876 (P<.001), respectively), and between our model and that proposed by Figueras it was "substantial" (κ=0.720, P<.001).

The new model is comparable to those used for Spanish children and allows accurate, updated automatic percentile calculation for gestational age and sex. The results can be digitally stored to track longitudinal foetal growth. Free access to the model is offered, together with the possibility of automatic calculation of foetal and neonatal weight percentiles.