Birth weight estimates updated

A research led by the Harvard Pilgrim Health Care Institute has provided an updated  reference charts for birth weight for gestational age, used to determine the health of newborns, using a nationally representative sample of birth data and obstetric estimates of gestational age. This update could better reflect the current socio-demographics of the US population and identify infants who need additional care, according to researchers in the US.  

“Previous birth weight charts may not reflect the current sociodemographic composition of the US,” explains Izzuddin Aris, who led the new study. “In addition, they rely on gestational age estimated from maternal reports of last menstrual period, which is often less accurate than obstetric estimates that take into account ultrasound measurements, menstrual history, and laboratory tests.”

Birth weight is an indicator of health for a baby and its mother, so it is important to know whether a newborn’s weight is healthy for the stage of pregnancy at which the infant is delivered, also known as gestational age. A birth weight at the extremes of the range for a given age indicates restricted or excessive growth and is associated with health risks.

Reference charts are used to determine whether an infant’s birth weight is within the normal range. These charts indicate the proportion of infants born within ranges of weight at different gestational ages. However, current birth weight charts used in the US are based on old data, and the standard approach of estimating the gestational age relying only on the mother’s reports of her last period.

Aris and colleagues developed new birth weight reference charts based on the birth weights of more than 3.8 million infants born in the US in 2017 at gestational ages of 22–42 weeks, according to obstetric estimates. They then applied two modelling techniques to construct curves that show average birth weights of boys and girls and the percentage of infants within various weight ranges at different gestational ages.

The two modelling techniques produced curves that overlapped considerably, indicating the validity of the approaches. Comparison of the new charts with charts from 2009 and 1999 revealed considerable differences. For infants born preterm, the new charts classified fewer infants as small, and more infants as large, for their gestational age. For infants born post-term, the new charts classified more infants as small, and fewer as large. The differences probably result from changes in socio-demographics and the accuracy of obstetric estimates of gestational age.

“We expect clinicians to be able to use the updated reference to identify at-risk infants in need of additional monitoring or care,” says Aris. “The next step is to compare existing birth weight standards with our updated reference in predicting important health outcomes later in life.”

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