OBJECTIVE: To provide a statistically sound criterion for identifying implausibly large birthweights for gestational age. DESIGN: Review of ISTAT 1990-1994 national newborn records. SETTING: Italy POPULATION: Forty-two thousand and twenty-nine single first and second liveborn preterm babies. METHODS: Two-component Gaussian mixture models are used to describe the birthweight distributions stratified by gestational age. Implausibly large babies are identified through model-based probabilistic clustering. MAIN OUTCOME MEASURES: Gestational age misclassification and weight-for-gestational age centile curves RESULTS: Gestational age appears under-estimated by about six weeks in 12.3% of the cases. Large babies are equally present in males and females, but are more frequent in second-borns than in first-borns, even when parity-specific models are fitted. CONCLUSIONS: The approach allows for a quantification of the gestational age under-estimate error and for data correction through model-based clustering. Correct birthweight distributions and growth curves are also provided.
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