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COMMENTARY |
Departments of Obstetrics and Gynecology, University of Vermont College of Medicine, Burlington, VT 05401-1435
It has been clear for some time that small newborns are members of 1 of 2 groups: those born too early and those who are small for their age. This distinction was formalized by the designations assigned by the World Health Organization; specifically, "premature and light for dates" as the 2 diagnoses for small newborns.1,2 For the most part, we have sought to segregate these 2 etiologies for smallness and have come to recognize distinct patterns of morbidity and mortality that attend them.
In this month’s issue of Pediatrics, Zaw et al3 have shown that our assessment of the contribution of poor fetal growth to newborn morbidity and mortality is influenced by the definitions we apply. At least 2 distinct methods for defining small newborn size for gestational age exist. The most common uses birth weight data as the standard. Alternatively, ultrasound estimates of fetal weight can be used to define normal size at each gestational age. The fact that these differ substantially should come as no surprise. Systematic limitations were identified shortly after the first birth weight standards were published that obscured their reflection of normal fetal growth.4–6 Several investigators have confirmed that the link between preterm birth and fetal growth restriction is paramount among them.7–9 When only 2% of the population is represented in a sample (ie, delivery before 32 weeks’ gestation), and only 9% to 10% before 37 weeks, it is easy for selection bias to skew the birth weight distributions such that they do not accurately represent standards for the normal population.
This problem is not simply about where to draw the line across growth curves that have similar shape. The relationship between poor fetal growth and preterm delivery is not consistent across preterm gestation (<37 weeks’ gestation).10 The differences between birth weight and estimated fetal weight are most pronounced, as a percentage of weight, between 27 and 29 weeks and the difference remains >10% between 26 and 31 weeks.10 These differences point to unique growth curves that converge before 26 and after 31 weeks making simple swapping of percentile thresholds across preterm gestation inappropriate. Nor is it a problem with estimates of fetal weight. Although the 95% confidence limit of any individual estimate of fetal weight is ±15%, these errors are nonsystematic and the estimate of the mean population fetal weight given a large sample is quite accurate.11
With 
25% of newborns <34 weeks’ gestation classified as growth-restricted, it becomes difficult to segregate the morbidity associated with growth restriction from that observed in the average preterm neonate. The data from the present study support the conclusion that fetal growth restriction contributes significantly to the overall burden of illness observed in premature infants. Additionally, the present study specifically suggests that those at increased risk for respiratory distress syndrome, bronchopulmonary dysplasia, retinopathy of prematurity, and intraventricular hemorrhage are more accurately identified when using fetal rather than neonatal growth standards. Whether the 10th percentile of fetal growth, as utilized in this study, is the optimal threshold for the segregation of those at increased risk for newborn morbidity remains to be determined. As suggested by the results provided it is likely that specific morbidities will have specific thresholds that best identify the populations at risk.
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FOOTNOTES |
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Address correspondence to Ira M. Bernstein, MD, Obstetrics and Gynecology, University of Vermont College of Medicine, Burgess 217, Fletcher Allen Health Care, 111 Colchester Ave, Burlington, VT 05401-1435. E-mail: ibernste{at}zoo.uvm.edu
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This article has been cited by other articles:
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  B. D Gessner and M.-A. R Chimonas Asthma is associated with preterm birth but not with small for gestational age status among a population-based cohort of Medicaid-enrolled children <10 years of age Thorax, March 1, 2007; 62(3): 231 - 236. [Abstract] [Full Text] [PDF]
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 E Bertino, S Milani, C Fabris, and M De Curtis Neonatal anthropometric charts: what they are, what they are not Arch. Dis. Child. Fetal Neonatal Ed., January 1, 2007; 92(1): F7 - F10. [Full Text] [PDF]
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