Errors in registered birth weight and its implications for mortality statistics.

BACKGROUND Birth weight mortality statistics are important for examining trends and monitoring the outcomes of neonatal care. AIM To determine the effects of errors in the registered birth weight on birth weight specific mortality. METHODS All twins born in England and Wales during 1993-95 comprise the denominator population. For those twins that died, the Office for National Statistics (ONS) provided copies of the death certificates. From the information on the death certificates, the registered birth weight was validated and amended using predetermined rules. The neonatal, postneonatal, and infant mortality rates were recalculated. RESULTS In 2.5% of cases the registered birth weight was "not stated" and in others there were miscoding errors. Important differences between published and amended birth weight specific mortality rates especially in <500 g and >/=3500 g groups were evident. CONCLUSIONS The bias arising from these errors should be taken into account in interpreting mortality rates and their trends.