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A Long TermSocial Investment. New. York, NY: Ford Foundation; 1989. 12. Lazar I, Darlington B. ..... NewYork: OxfordUniversityPress;. 1989. 5. Hammes LR ...
Editals

its children. A search for social interventions that work on behalf of children's

health yields rich supportive evidence. What fails to work is indifference to that evidence. What will work is a mobilized national will to help children and their families. It should be an important part of any national program to reform systems of health care financing and access. El C. Arden Miler, MD C. Arden Miller is with the University of North Carolina at Chapel Hill, School of Public Health. Parts of this editorial are taken from a paper the author prepared for a conference, "Medical Care and the Health of the Poor," held at Cornell University Medical Center in February 1992. The author thanks Dr. William Pratt of the National Center for Health Statistics and Dr. Lorraine Klerman of Yale University for helpful suggestions on a draft of this paper. Requests for reprints should be sent to C. Arden Miller, MD, Professor of Maternal and Child Health, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7400.

References 1. Werner EE, Bierman JM, French FE. The Children ofKauai Honolulu, Hawaii: University of Hawaii Press; 1971. 2. Williams LB, Pratt WF. Wanted and unwanted childbearing in the US, 1973-1988. Advance Data, National CenterforHealth Statistics. 1990;189:1-8. 3. Gold RB, Daley D. Public funding of contraceptive, sterilization, and abortion service, fiscal year 1990. Fam Planning Perspect. 1991;23(5):204-211. 4. Henshaw SK, Van Vort J. Abortion services in the United States, 1987 and 1988. Fam Plannung Perspect. 1990;22(3):102-

108,142. 5. Waller AE, Baker SP, Szocka A. Childhood injury deaths: national analysis and

geographicvariations.AmrJPublicHealth. 1989;79:310-315. 6. Select Committee on Children, Youth, and Families, US House of Representatives. Abused Children in Arneica: Vctins of Official Neglect. Washington, DC: Govt Printing Office; 1987. 7. Zuravin SJ. Unplanned childbearing and family size: their relationship to child ne-

glect and abuse. Fam Planning Perspect. 1991;23(4):155-161. 8. National Center for Health Statistics. Health Aspects of Prnancy and Childbirth, United States 1982. Hyattsville, MD: National Center for Health Statistics; 1988. US Dept of Health and Human Services publication PHS 89-1992. 9. Kagan SL, Zigler EF. Chapter 12. Early schooling: a national opportunity? In: Kagan SL, Zigler EF, eds. Eary Schooling the National Debate. New Haven, Conn: Yale University Press; 1987. 10. McKey RH, Condelli L, Ganson H, et al. The Impact of Head Start on Children, Families, and Comnumitis. Washington, DC: Govt Printing Office; 1985. US Dept of Health and Human Services publication OHDS 85-31193. 11. Weikart DP. Quality PreschoolProgrms: A Long Term Social Investment. New York, NY: Ford Foundation; 1989. 12. Lazar I, Darlington B. Lasting Effects of Early Educaton. A Report from the Consortium for Longitudinal Studies. 1989;47(2-3):1-66. Monograph Soc. Res. Child Den.

Preterm Birth, Intrauterine Growth Retardation, and Perinatal Mortaity The high mortality rate among infants born too small is now a phenomenon well known to educated Americans. What is less well known is that the low birthweight rate (the proportion of live births weighing less than 2500 g) has changed little in the last few decades. In 19 years (1970 to 1988), the low birth weight rate for the United States declined 13%, from 7.93% to 6.93%. Over the same period, the very low birth weight rate (the proportion of live births weighing less than 1500 g) actually increased from 1.17% to 1.24%.1 The stability of these rates challenges perinatal epidemiologists; it is one manifestation of the difficulty in identifying preventable causes of low birth weight, despite years of research effort. Intractable problems of this sort tend to elicit two general responses. One is to generate and test new research hypotheses. Over the last few decades, for example, hypotheses concerning prenatal care, maternal nutrition, cigarette smoking, genito-urinary tract colonization and infection, work and physical activity during pregnancy, psychosocial stress, occupational stress, alcohol consumption, drug use, and maternal anemia have been creatively tested.2- 4A second response is methodological for example, to improve measurement of relevant variables, to refine the classification of low birth weight

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babies, and to apply innovative techniques of quantitative analysis. The 20th century history of the measurement of birth weight and gestational age changed as concepts and knowledge changed;4 it can be divided roughly into the following overlapping phases: 1. The common use of "prematurity" as an entity defined by birth weight alone, usually 2500 g or less (1920 to 1980). 2. The recognition that birth weight and gestation are far from perfectly correlated and that both data are useful in assessing prognosis for newborns (1950s to present). 3. Common use, as the measure of gestation, of the interval from the date of the last menstrual period (LMP) to the date of delivery (1960s to the present). In the pre-ultrasonography era, gestation based on LMP proved to have greater statistical and biological validity than gestation based on physician's estimates.5 4. The recognition that the low birthweight population consists of two major groups-preterm infants and intrauterine growth-retarded infants (1960s to the present). Preterm delivery has usually (if inconsistently) been defined as less than 37 completed weeks of gestation. Intrauterine growth retardation (IUGR) has been preferentially defined by obstetric and pediatric clinicians as below the 10th percentile of birth weight for gestational age.

5. The measurement of weight for gestational age on a continuous scale,6 which yields a measure of the degree of IUGR among those affected7 (1980 to the present). 6. The subdivision of infants with IUGR according to symmetry of body proportions in terms ofweight, length, and head circumference (early 1970s to the present). In this conceptualization, "disproportionally" or "asymmetrically" growth-retarded infants have normal length and head circumference for gestational age but low birth weight, and hence a low index of weight for length. "Proportionally" or "symmetrically" growth-retarded infants are equally retarded in weight, length, and head circumference. 7. Reinforcement of the centuriesold view that the estimation of gestational age from LMP can be fraught with error, particularly in registration data such as birth certificates. 8. The advent of prenatal ultrasonic examinations (circa 1970). Especially when it is impossible to gather detailed information on menstrual histories, early ultrasonography may give more accurate estimates of age than those based on LMP.4

Editor's Note. See related article by Wilcox and Skjaerven on page 378 of this issue. American Journal of Public Health

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9. Acknowledgment that ultrasonic scanning measures size,8 not fetal age (1985 to present) and that further validation is required. Some researchers have, therefore, constructed decision rules9'10 that rely on sonographic estimates only when the LMP date is unknown or implausible. This brings us to the interesting, even provocative, report by Wilcox and Skjaerven11 in this issue of the Journal. The authors analyze the association of perinatal mortality with the continuous variables, gestational age and birth weight. For practitioners concerned with prevention and treatment, to speak in terms of categories or entities, namely preterm birth and IUGR, lends concreteness to research results. Preterm birth and IUGR often occur together, but IUGR in full-term births yields an exclusive category. Preterm birth is much the greater contributor to perinatal mortality than is full-term IUGR, especially if fatal congenital malformations are excluded.12-14 Nevertheless, IUGR is also a strong risk factor for perinatal mortality.7'14 The more severe the IUGR, the higher the mortality; within narrow gestational age intervals, mortality increases exponentially as birth weight decreases from the median15 or modal16 weight for gestation. A brief comment on preventability is in order here. Wilcox and Skjaervenll assert that preterm delivery is "perhaps more tractable to change" than low birth weight. The evidence suggests otherwise. In a comprehensive review of the determinants of low birth weight, Kramer3 concludes that the prevalence of IUGR could be reduced by up to two thirds if maternal cigarette smoking, low caloric intake or low weight gain, and low prepregnancy weight were eliminated. By contrast, current knowledge of risks leaves nearly three quarters of preterm births unexplained. Changes over time in the US birth weight distribution, insofar as they reflect prevention, or at least malleability, are consistent with this understanding. Moderately low birth weight (1500 to 2499 g) rates include term IUGR infants and have declined; very low birth weight rates include preterm birth almost exclusively and remain unchanged.1 Wilcox and Skjaerven11 applied the method of Wilcox and Russell17 to derive standardized (z) scores for birth weights relative to a Gaussian mean within each gestational age interval. From the results of this analysis of "relative size" they choose to emphasize the role of gestational age in perinatal mortality. Perinatal 344 American Journal of Public Health

mortality relates strongly to the size of the infant relative to the mean at a given gestational age interval. Thus, we are inclined to read the paper as demonstrating the role of intrauterine growth rate, while taking that of gestational age as given. The pattern points to a strong association of intrauterine growth with perinatal mortality. Within each gestation interval, perinatal mortality increases exponentially as birth weight decreases from the mean weight for that gestation or, more precisely, the mean of the "predominant distribution"17 for that gestation (see Figure 3 on p. 381).11 The result is consistent with more direct approaches than the Wilcox-Russell method, namely, mortality rates plotted by birth weight percentile within each week of gestation.15,16 There is no question that gestational age is a powerful predictor of perinatal survival: perinatal mortality is high among preterm births.12-14,18 The question that does arise from this paper is whether the Wilcox and Skjaerven analysis reveals more than the "usual" analysis of birth weight, gestation, and perinatal mortality,2'4'19'20 properly interpreted. As the authors11 point out, the demonstration that birth weight predominates as a correlate of perinatal mortality when birth weight and gestation are analyzed together24'19'20 can be misinterpreted to mean that "birth weight is fundamentally more important to survival than gestational age." A more sensible interpretation would be that (1) in preterm infants, short gestation precludes adequate growth and development and results in small size and low birth weight; (2) this limit on growth, development, and size underlies much of the high mortality of preterm infants; (3) low birth weight is consequently a very accurate indicator of mortality risk. It has been asserted (and contested) that the Wilcox-Russell method can provide a deeper understanding ofdifferences between racial and ethnic groups in infant mortality.21'22 Others have thought the method to be a valuable adjunct in international and interstate23 comparisons of birth weight-specific infant mortality rates. Certainly, the Wilcox-Russell mode of adjustment provides an alternative perspective on the relations of gestational age, birth weight, and mortality. The problem of logical interpretation remains. No inherent structure can be assigned to the several possible forms of statistical analysis. The choice of an appropriate model is determined by collateral evidence, developmental and time sequences in the processes observed, and

the general coherence of results. Internal to the analysis, no grounds for a decisive choice exist. Does the Wilcox-Russell method improve understanding of the particular relationships of perinatal mortality to biological variables such as gestational age and birth weight? In this instance, the results of more straightforward analyses lead to much the same conclusions: both birth weight and gestational age are important in perinatal mortality.12-1618,225 Nor should this new analysis detract from birth weight as an invaluable operational index in epidemiology. This is not to pass over the deficiencies in current knowledge. Perinatal researchers need to invest energy in improving the measurement of gestational age; we need to bring biology to bear in developing and testing new hypotheses about the pathogenesis of preterm labor and IUGR, and we need to refine our understanding of the influence of social position as it interlocks with race and ethnicity.4,21 [] John L. Kiely, PhD Mervyn Susser, MB, BChJ, FRCP(E), DPH John L. Kiely is with the National Center of Health Statistics. Mervyn Susser, at the time of this writing, was Director of the Gertrude H. Sergievsky Center, Columbia University. He is now the Editor of the Journal. Requests for reprints should be sent to John L. Kiely, PhD, Division of Analysis, National Center for Health Statistics, 6525 Belcrest Road, Hyattsville, MD 20782.

References 1. National Center for Health Statistics. Health, United States, 1990. Hyattsville, Md: Public Health Service; 1991:58. 2. Thomson AM. Fetal growth and size at birth. In: Barron SL, Thomson AM, eds. Obstet EpidenioL New York: Academic Press; 1983:89-142. 3. Kramer MS. Determinants of low birth weight: methodological assessment and meta-analysis. Bull WHO. 1987;65:663737. 4. Kline J, Stein Z, Susser M. Conception to Birth Epidemiology of Prenatal Development. NewYork: OxfordUniversityPress; 1989. 5. Hammes LR, Treloar AE. Gestational interval from vital records. Am J Public Health. 1970;60:1496-1505. 6. Bland JM, Peacock JL, Anderson HR, et al. The adjustment of birthweight for very early gestational ages. Appl Stat.

1990;39:229-239.

7. Kramer MS, Olivier M, McLean FH, et al. Impact of intrauterine growth retardation and body proportionality on fetal and neo-

natal outcome. Pediatrics 1990;86:707-713. 8. Hall MH. Definitions used in relation to gestational age. Paediatr Pernnat EpidemioL 1990;4:123-128.

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Editorials 9. Hall MH, Carr-Hill RA, Fraser C, Samphier ML. The extent and antecedents of uncertain gestation. Br J Obstet GynaecoL 1985;92:445-451. 10. Wood C. Accurately dating pregnancy. In: Merkatz IM, Thompson JE, eds. New Per-

spectives on Prenatal Care. New York: Elsevier; 1990:123-134. 11. Wilcox AJ, Skjaerven R. Birth weight and perinatal mortality: The effect of gestational age. Am J Public Health. 1992;82:378-382. 12. Rush RW, Keirse MJNC, Howat P, et al. Contribution of preterm delivery to pennatal mortality. Br Med J. 1976;2:965-968. 13. Rush RW, Davey DA, Segall ML. The effect of preterm delivery on perinatal mortality. Br J Obstet GynecoL 1978;85:806811. 14. Gruenwald P. Growth and maturation of the foetus and its relationship to perinatal mortality. In: Butler NR, Alberman ED, eds. Pennatal Pmbems: 7he Second Report of

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o the 1958 British P talM Mflity Swvey. Baltimore, Md: Williams & Wilkins; 1969:141-162. Williams RL. Creasy RK, Cunningham GC, et al. Fetal growth and perinatal variability in California. Obstet Gynecol. 1982;59:624-632. Myers SA, Ferguson R. Apopulation study of the relationship between fetal death and altered fetal growth. Obstet GynecoL 1989;74:325-331. Wilcox AJ, Russell IT. Birthweight and perinatal mortality: III. Toward a new method of analysis. Internat J Epidemiol. 1986;15:188-196. Hoffman HJ, Meirek 0, Bakketeig LS. Methodological considerations in the analysis of perinatal mortality rates. In: Bracken MB (ed.). Perinatal Epidemiology. New York, NY: Oxford University Press; 1984:491-530. Bergner L, Susser MW. Low birthweight and prenatal nutrition: An interpretive review. Pediatrics. 1970;46:946-966.

20. Susser M, Marolla FA, Fleiss J. Birth weight, fetal age and perinatal mortality. Am JEpidemioL 1972;96:197-204. 21. David R. Race, birth weight, and mortality rates. JPedJ 1990;116:101-102. 22. Chen R, WaxY, LuskyA, et al. A criterion for a standardized definition of low birthweight. IntematJEpidemioL 1991;20:180186. 23. Kleinman, JC. Methodologic issues in the analysis ofvital statistics. In: Kiely M, ed. Reproductive and Peninatal Epidemiology. Boca Raton, Fl: CRC Press; 1991:447-468. 24. Fedrick J. Comparison of birthweight/ gestation distribution in cases of stillbirth and neonatal death according to lesions found at necropsy. Br Med J. 1969;3:745748. 25. Karlberg P, Priolisi A, Landstrom T, Selstam U. Clinical analyses of causes of death with emphasis on perinatal mortality. MonogrPaediat. 1977;9:86-120.

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