%0 Journal Article
%T Thinking outside the curve, part I: modeling birthweight distribution
%A Richard Charnigo
%A Lorie W Chesnut
%A Tony LoBianco
%A Russell S Kirby
%J BMC Pregnancy and Childbirth
%D 2010
%I BioMed Central
%R 10.1186/1471-2393-10-37
%X We propose describing a birthweight distribution via a normal mixture model in which the number of components is determined from the data using a model selection criterion rather than fixed a priori.We address a number of methodological issues, including how the number of components selected depends on the sample size, how the choice of model selection criterion influences the results, and how estimates of mixture model parameters based on multiple samples from the same population can be combined to produce confidence intervals. As an illustration, we find that a 4-component normal mixture model reasonably describes the birthweight distribution for a population of white singleton infants born to heavily smoking mothers. We also compare this 4-component normal mixture model to two competitors from the existing literature: a contaminated normal model and a 2-component normal mixture model. In a second illustration, we discover that a 6-component normal mixture model may be more appropriate than a 4-component normal mixture model for a general population of black singletons.The framework developed in this paper avoids assuming the existence of an interval of birthweights over which there are no compromised pregnancies and does not constrain birthweights within compromised pregnancies to be normally distributed. Thus, the present framework can reveal heterogeneity in birthweight that is undetectable via a contaminated normal model or a 2-component normal mixture model.The impact of birthweight on perinatal mortality and morbidity has been debated for decades [1-11]. Although advances in maternal and perinatal care have reduced overall mortality, infants with very low birthweights (1000-1500 g; VLBW) and extremely low birthweights (<1000 g; ELBW) remain at high risk. These infants require more intensive utilization of health resources, at increased costs relative to normal birthweight (NBW; 2500-4000 g) infants [12-14]. Even infants of moderately low birthweight (1500-25
%U http://www.biomedcentral.com/1471-2393/10/37