Construction of the birth weight by gestational age population reference curves of Catalonia (Spain): Methods and development.

Infant size at birth is a useful indicator to evaluate fetal growth in relation to gestational age. There is no standard model to create anthropometric reference curves in neonates, but the method chosen could determine the reference values estimated. We describe the methods used to construct population-based reference curves of birth weight for gestational age in Catalonia, Spain. These methods included detection of implausible values of birth weight for gestational age by a probabilistic cluster model, utilization of the Generalized Additive Model for Location and Scale method to obtain smoothed percentiles and z-scores, and calculation of 95% confidence intervals by bootstrapping. To our knowledge, these are the first reference curves in neonates constructed through a method allowing asymmetric distributions with kurtosis to be modelled. Estimation of confidence intervals is useful to determine which reference intervals can be employed to assess newborn size.