Parametric modelling of neonatal mortality in relation to size at birth

We use a richly parameterized model to analyse the effects of shortness and thinness at birth on neonatal mortality in Swedish live-born singletons. T he model captures the hazard of neonatal mortality according to the functio n alpha x exp(-gamma x t) + delta, where t represents age. Covariates are a llowed to simultaneously influence the initial excess hazard, the rate of d ecay, and the long-term hazard. Among term newborns, birth length for gesta tional age had a stronger effect on the long-term hazard and a weaker effec t on the initial excess hazard than the Ponderal index. The initial excess hazard associated with a low Ponderal index tended to decay quickly. Among preterm newborns, a higher birth length for gestational age was associated with lower initial and long-term hazards, and with a faster rate of decay o f the initial excess hazard. In contrast, the Ponderal index was not associ ated with the long-term hazard. We discuss the interpretability of the mode l and its potential use in neonatology and medical demography. We also comp are the model to a Cox model with time-dependent covariates. Copyright (C) 2001 John Wiley & Sons, Ltd.