Ponderal index of the newborn: effect of smoking on the index of the small-for-gestational-age infant.

The ponderal index (PI) is evaluated in the context of its distribution within a given population. Low PI (<10th percentile for gestational age) has been extensively studied but not much is known about the distribution and factors associated with a high (>90th percentile) PI among small-for-gestational-age (SGA) infants. This retrospective study of singleton live first births from 1990 to 2007 in a Midwestern city explores factors associated with a high PI, particularly among SGA infants. Independent variables included exposures (none, smoke, hypertension) and maternal and infant demographic characteristics. There were 45,453 births, 28 to 42 weeks' gestational age, 55% Whites and 51% male. Mean PI increased with gestational age and was highest among Hispanics and lower among SGA infants. High PI was present in 11% of appropriate-for-gestational-age and 4% of SGA infants. Among SGA infants, odds ratios (ORs) of high PI were higher for smoke exposure (1.21; 95% confidence interval 0.97, 1.87) and lower for males (0.66; 0.47, 0.93). In conclusion, the distribution of PI varies by exposures and of high PI by race/ethnicity/gender. SGA infants with high PI have relative surplus of mass, and ostensibly, adiposity, for their frame. There is a need to use PI in exploring and defining previously observed associations between SGA and adult-onset obesity/metabolic syndrome.