Customized birth weight for gestational age standards: Perinatal mortality patterns are consistent with separate standards for males and females but not for blacks and whites

Abstract

BACKGROUND: Some currently available birth weight for gestational age standards are customized but others are not. We carried out a study to provide empirical justification for customizing such standards by sex and for whites and blacks in the United States. METHODS: We studied all male and female singleton live births and stillbirths (22 or more weeks of gestation; 500 g birth weight or over) in the United States in 1997 and 1998. White and black singleton live births and stillbirths were also examined. Qualitative congruence between gestational age-specific growth restriction and perinatal mortality rates was used as the criterion for identifying the preferred standard. RESULTS: The fetuses at risk approach showed that males had higher perinatal mortality rates at all gestational ages compared with females. Gestational age-specific growth restriction rates based on a sex-specific standard were qualitatively consistent with gestational age-specific perinatal mortality rates among males and females. However, growth restriction patterns among males and females based on a unisex standard could not be reconciled with perinatal mortality patterns. Use of a single standard for whites and blacks resulted in gestational age-specific growth restriction rates that were qualitatively congruent with patterns of perinatal mortality, while use of separate race-specific standards led to growth restriction patterns that were incompatible with patterns of perinatal mortality. CONCLUSION: Qualitative congruence between growth restriction and perinatal mortality patterns provides an outcome-based justification for sex-specific birth weight for gestational age standards but not for the available race-specific standards for blacks and whites in the United States.

Figure 1

Gestational Age and Birth Weight Distributions of Male and Female Singleton Live Births. Gestational age (1a) and birth weight (1b) distributions of male and female singleton live births ≥22 weeks and ≥500 g in the United States, 1997 and 1998.

Figure 2

Conventional Calculation: Birth Weight- and Gestational Age-Specific Perinatal Mortality Rates among Male and Female Births. Conventional calculation: birth weight-specific (2a) and gestational age-specific (2b) perinatal mortality rates per 1,000 total births among male and female singleton births in the United States, 1997 and 1998.

Figure 3

Fetuses at Risk Approach: Gestational Age-Specific Birth, Labor Induction and Perinatal Mortality Rates among Male and Female Births. Fetuses at risk approach: Gestational age-specific birth rates (3a, primary Y-axis), labor induction rates (3b, primary Y-axis) and perinatal mortality rates (3a and 3b, secondary Y-axis) among male and female singleton births in the United States, 1997 and 1998.

Figure 4

Fetuses at Risk Approach: Gestational Age-Specific Growth Restriction and Perinatal Mortality Rates among Male and Female Births. Fetuses at risk approach: Gestational age-specific fetal growth restriction (primary Y-axis) and perinatal mortality rates (secondary Y-axis) among male and female singleton births, with growth restriction rates based on sex-specific (4a) and unisex (4b) birth weight for gestational age standards, United States, 1997 and 1998.

Figure 5

Fetuses at Risk Approach: Gestational Age-Specific Fetal Growth Restriction and Perinatal Mortality Rates among White and Black Births. Fetuses at risk approach: Gestational age-specific fetal growth restriction (primary Y-axis) and perinatal mortality rates (secondary Y-axis) among white and black singleton births, with growth restriction rates based on a race-specific standard (5a) and on a single birth weight for gestational age standard (5b), United States, 1997 and 1998.