The number of births in teenage mothers was higher in immigrant mothers from all origins than in Spanish women. The highest rates of low birth weight and preterm births were found in women from eastern Europe and sub-Saharan Africa.

Our results suggest some limitations in the quality of the data on gestational age and birthweight provided by NPM data, especially for comparisons of preterm and low birthweight indicators in the Spanish-born and immigrant populations. Moreover, the results point to the quality of the congenital metabolic disorders registers as a source to compare reproductive and perinatal health indicators.

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.

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