The dilemma of less than 500 grams birth: epidemiologic considerations.

The reporting practice of perinatal data of less than 500 gm is widely inconsistent. This is due mainly to the differences in reporting requirements and ambiguity in the definition of live births, stillbirths and abortions. To evaluate the magnitude a birthweight of less than 500 gm has on a regional perinatal network's vital statistics, we studied race and weight specific data from a cohort of 48,096 births over a 2-year period. One hundred and ten (0.23%) births weighed less than 500 gm, 60 of them were live births occurring at a mean gestation of 22.3 +/- 3 weeks, all of whom died. Blacks had the highest rates for other adverse perinatal outcome measures as well as significantly higher incidence of birthweights less than 500 gm (0.3%), compared to whites and Hispanics, 0.17 and 0.21%, respectively (P less than 0.001). Although they constituted a small fraction of the total births in all ethnic groups, infants weighing less than 500 gm accounted for a significant proportion, 18 and 21%, of the perinatal and neonatal mortality rates (PMR and NMR). These proportions were much higher in the black subgroup. We conclude that major discrepancies in reporting of vital data concerning these infants could affect the vital rates significantly. The effect is not uniform, since the subsets with a high incidence of infants weighing less than 500 gm will have far more variations. This must be considered when comparative epidemiologic studies of perinatal outcome are carried out.