Rohrer's ponderal index (birth weight 3 100/CHL^3) is often used to categorize clinical risks such as SGA and LGA and to compare mortality rates [Lubchenco et al Pediatrics 1966;37:403-8]. Per Lubchenco, “There is an increasing weight-length ratio (viz: the ponderal index) as gestation proceeds: the babies become heavier for length as they near full term (p. 407). Variations in measuring crown-heel length are minimized by large databases; Lubchenco et al compared 4,706 liveborn PI over 26-42 weeks of gestation. The present study involved birth weights and birth CHL from 530 perinatal autopsies; the CHL for newborns up to 10 days of life were verified by autopsy CHL. The plot of PI against gestation by percentile not only failed to show any progression but was a series of essentially parallel lines with minor variations and then a steady decline in the 100th percentile from 32 to 43 weeks. The most divergent points were at 28-32 weeks at higher percentiles, 90-100. This effect was shown by Lubchenco and similar effects were seen in mortality rates for high birth rates (3,000+ g) at 28-32 weeks [Erhardt et al. Am J Public Health 1964;54:1841-55]. The different range of PI in stillborns and neonatal deaths bespeaks of a distinct developmental biology, which may carry pathogenetic potential (heavier is "bad "). The displacement of PI, both empiric and by percentile, over the interval of 28-32 completed weeks, points to a second factor or net factors of significance in growth and development.
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