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Should we adjust for gestational age when analysing birth weights? The use of z-scores revisited
Ilse Delbaere1, Stijn Vansteelandt, Dirk De Bacquer
1Department of Obstetrics and Gynaecology, Ghent University, De Pintelaan 185, B-9000 Ghent, Belgium. ilse.delbaere@ugent.be
Insights
Birth weight is a key indicator of infant health. Adjusting birth weight for factors like gestational age may introduce bias, even when using z-scores or nomograms.
Area of Science:
- Reproductive Medicine
- Neonatal Health
- Biostatistics
Background:
- Birth weight is a critical predictor of neonatal and infant mortality and morbidity.
- Birth weight varies significantly across populations due to demographic factors and differential fetal growth.
- Standardizing birth weight using population-specific nomograms and z-scores is common practice.
Purpose of the Study:
- To critically evaluate the practice of adjusting birth weight for factors that may lie on the causal pathway.
- To highlight the potential for induced bias when standardizing birth weight using methods like z-scores or nomograms.
Main Methods:
- Review of statistical methodologies for birth weight standardization.
- Analysis of potential biases introduced by adjusting for covariates on the causal pathway.
- Discussion of the implications of using z-scores and nomograms for birth weight assessment.
Main Results:
- Adjustment for factors such as gestational age, which are on the causal pathway between conception exposures (e.g., single-embryo transfer vs. double-embryo transfer) and birth weight, can induce bias.
- This bias can occur irrespective of the adjustment method used (stratification, regression, or z-scores).
Conclusions:
- Standardizing birth weight by adjusting for gestational age may obscure true effects and lead to biased interpretations.
- Rethinking the routine adjustment of birth weight is necessary to accurately assess neonatal outcomes and risk factors.
Abstract:
Birth weight is the single most important risk indicator for neonatal and infant mortality and morbidity, which has led to the idiom that 'every ounce counts'. Birth weight in turn, however, tends to vary widely across populations as a result of differential fetal growth velocity with such demographic factors as ethnicity, maternal and paternal height and altitude of residence. Accordingly, it has been acknowledged that the appraisal of birth weight should rely on its position relative to the birth weight distribution of the background population. This is commonly done by standardizing birth weight through its deviation from the population mean in the given gestational age stratum, as can be obtained from population-customized birth weight nomograms. This issue was recently revisited in 'Human Reproduction' through a plea for reporting birth weight as z-scores. In this article, we argue that adjustment for factors, such as gestational age, which may lie on the causal pathway from exposures present at the time of conception [e.g. single-embryo transfer (SET) versus double-embryo transfer (DET)] to birth weight, may induce bias, regardless of whether the adjustment happens via stratification, regression or through the use of z-scores.
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