An Updated Birth Weight Reference For All Babies Big, Small And Everything In Between

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Izzuddin Aris, PhD

Fetal growth, typically measured as birth weight-for-gestational-age, predicts morbidity, survival, and long-term health outcomes in children and their mothers. Physicians typically use percentile thresholds (for example, the 10th or 90th percentile) from population-based reference charts to identify at-risk infants who may have had restricted or excessive fetal growth. Children identified as being small or large for their gestational age often need more care and careful monitoring during early life.

​Previous birth weight charts however, suffer for two reasons. First, they may not reflect the current socio-demographic composition of the United States. And, second, they rely on very uncertain estimates of gestational age, from maternal reports of their last menstrual period. This subjective dating method is often less accurate than obstetric estimates based on ultrasound measurements, menstrual history and laboratory values. Given these concerns, we need a new and improved obstetric-estimate-based reference birth weight-at-gestational age.

In our latest study, we have created an updated birth weight reference for both clinicians and researchers using the most recent and nationally representative data on birth weight and obstetric estimates of gestational age. We used publicly available birth certificate data on over 3 million live births from 2017 to develop reference percentile curves and gestational age-specific cutpoints for male and female infants as well as for first-, second- and third-or-more-born infants. This data had the more precise obstetric measures and was more representative of the entire US population. We also created a simple and easy to use online tool for both clinicians and researchers to quickly calculate measures of birth size, tailored to their specific needs. To give an example, a female infant born at 3000 grams (~6 pounds, 9 ounces) at 38 weeks’ gestation would be in the 35th percentile using our updated reference. The same infant, however, would be classified in the 29th percentile using an older reference that was based on live births in 1999‒2000.
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How should these results be used? We expect clinicians to be able to use the percentile thresholds from the updated reference to identify at-risk infants in need of additional monitoring or care. Researchers may also use the reference to derive continuous measures of birth size for studies examining predictors of fetal growth or relationships of fetal growth with later health outcomes. Previously published birth weight references, such as those by Oken et al, have been readily adopted in clinical settings. For example, Massachusetts General Hospital in Boston currently uses the “Oken reference” as a growth standard for all fetal ultrasounds. We hope that this updated reference will have a similar uptake.