For more than 80 years, the American Academy of Pediatrics has identified newborns as underweight if they weigh less than 5 and a half pounds.
But some researchers argue this one-size-fits-all approach ignores the fact that infants vary naturally in size based on their genes and environment.
St. Louis University researchers compared birth weights of siblings and found younger siblings who weighed at least a pound less than their older sibling at birth, were twice as likely to die in the first month of life.
Maya Tabet, professor of epidemiology at St. Louis University and co-author of the study, said the current threshold for identifying underweight babies may “conceal important differences in the risk of mortality.”
“What is optimal for one baby may not be the same as what’s optimal for another baby,” Tabet said.
Though it’s difficult to measure a baby’s ideal birth weight, the size of older siblings at birth is often a good predictor of a younger sibling’s birth weight.
That’s largely because you share half of your genes with your siblings.
In the study, Tabet and her colleagues at SLU identified nearly 180,000 pairs of siblings born in Missouri between 1989 and 2005.
When the siblings were similar in size at birth, the risk of dying in the first month of life, known as neonatal death, was low.
But younger siblings who weighed at least a pound less than their older siblings were more than twice as likely to die. The smaller they were compared to their older sibling, said Tabet, the higher their risk of death.
Perhaps even more tellingly, babies classified as average weight based on current recommendations still had a higher risk of dying if they weighed less than their older sibling.
“We’re missing some of those infants who have a birth weight above the current cut-off point, but who might still be at increased risk of dying in the first month of life,” Tabet said.
About one in 12 babies born in the U.S. is underweight, according to the Centers for Disease Control.
The rate in Missouri is slightly higher than the national average.
A complex mix of factors can affect a baby’s birth weight. The SLU team found Missouri mothers who gave birth to small babies were more likely to be African-American, low income and to receive inadequate prenatal care.
Taken as a whole, Tabet said the results may help doctors better identify at-risk newborns.
“It helps them to quantify the risk among these infants,” she said. “Therefore, it allows for more surveillance and possibly for earlier intervention.”
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