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J Epidemiol Community Health. 1996 October; 50(5): 559–563.
PMCID: PMC1060350

The undesirable consequences of controlling for birth weight in perinatal epidemiological studies.


OBJECTIVE: To compare the effects of controlling for birth weight with those of controlling for gestational age at delivery in perinatal epidemiological studies using two examples. SETTING: Western Australia. SUBJECTS: Population data: all white births born at 20-46 weeks of gestation in Western Australia during 1985-91 inclusive (n = 147564). Example 1: All Western Australian births from 1980-89 born either at 33-36 weeks inclusive (n = 13607), or born with a birth weight of 2050-2900 g (n = 34107). Example 2: 160 singleton cases of spastic cerebral palsy born to white mothers in Western Australia from 1975-80 and whose gestational age was known, compared with (a) 480 controls individually matched for gender and birth weight and (b) singletons with known gestational age liveborn to white mothers in Western Australia from 1980-81, or 1979-82 if < 30 weeks' gestational age at birth (n = 32031). MEASUREMENTS AND MAIN RESULTS: The risks of cerebral palsy associated with two separate exposures in groups defined by birth weight were compared with those in groups defined by gestational age. The origin of the differences are explained using total population data. The estimates of risk differ when exposure and outcome are both associated with appropriateness of fetal growth. The difference varied with gestational age, being greatest in the moderately preterm (33-36 weeks' gestation). CONCLUSION: Epidemiological studies in which appropriateness of fetal growth is an important variable should be based on gestational age at birth rather than birth weight, whatever the neonatal size or maturity.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.
  • Hall MH, Carr-Hill RA, Fraser C, Campbell D, Samphier ML. The extent and antecedents of uncertain gestation. Br J Obstet Gynaecol. 1985 May;92(5):445–451. [PubMed]
  • Blair E, Stanley F. Intrauterine growth and spastic cerebral palsy. I. Association with birth weight for gestational age. Am J Obstet Gynecol. 1990 Jan;162(1):229–237. [PubMed]
  • Kiely JL. Some conceptual problems in multivariable analyses of perinatal mortality. Paediatr Perinat Epidemiol. 1991 Jul;5(3):243–257. [PubMed]
  • Blair E, Stanley F. When can cerebral palsy be prevented? The generation of causal hypotheses by multivariate analysis of a case-control study. Paediatr Perinat Epidemiol. 1993 Jul;7(3):272–301. [PubMed]
  • DiPietro JA, Allen MC. Estimation of gestational age: implications for developmental research. Child Dev. 1991 Oct;62(5):1184–1199. [PubMed]

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