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Study finds low birth weight rates vary widely across US

November 07, 2005

Research may hold key to reduce national rate and improve neonatal health

HANOVER, NH - Low birth weight, an important risk factor of infant mortality and childhood developmental disorders, varies more than 3-fold in regions across the U.S., according to national research conducted at Dartmouth Medical School. The study offers promise for health care experts in an area of prenatal health where progress has been elusive.

Published in the November 7 issue of Pediatrics, the study is the first to investigate regional low birth weight rates on a national scale, and identifies regions that have significantly low or high low birth weight rates. The authors, based at the Center for the Evaluative Clinical Sciences at Dartmouth Medical School, concluded that birthplace is as important for neonatal outcomes as the race or prenatal health of the mother.

A low birth weight baby is defined as a newborn weighing less than 5.5 pounds. Although researchers have long known that low birth weight can be influenced by many factors including the biological interaction of the mother and the fetus, the parent's socioeconomic status, and medical care, these factors are little understood and public health initiatives aimed at reducing the incidence of low birth weight have been largely unsuccessful. National rates of low birth weight have actually increased over the past decade, a trend that has both economic and health consequences.

This study confirmed established knowledge that race plays a role in low birth weight, with an incidence of over 11 % of births to black mothers compared to less than 5 % of births to white mothers. Smoking or drinking during pregnancy approximately doubles the likelihood of low birth weight, as does a weight gain of less than 20 lbs. during pregnancy. Even after controlling for these established risk factors, the research team found that babies born in some regions of the U.S. were still more than 3 times as likely to be low birth weight compared to others.

"I was surprised that the regional variation across the country was still so high after accounting for other known factors," said Dr. Lindsay Thompson, who led the research at Dartmouth Medical School. "It is clear that place of residence is an important factor in neonatal outcomes," she said. "This is encouraging because these regions are linked to social and healthcare systems, and these are amenable to improvement."

Using a study population of 3,816,535 U.S. singleton births in 1998, and controlling for different characteristics that could predispose a child to being low birth weight, researchers then split the country into three groups that were above, the same, or below the national average of 6 low birth weight babies out of every 100 born. They found that of the 246 regions of the country used, 98 regions were higher than the national average and 67 were lower than the national rate by a significant degree. [see map]

While they do not know exactly why these regions vary to such a large degree, researchers hope that these data serve as a foundation for future studies and comparisons. They anticipate that regions with lower adjusted rates could serve as benchmarks for regions that need to improve through additional research and prevention efforts.

"The next research step is to look more closely at the types of available care and the services received by women in these regions," said co-author Dr. David Goodman, professor of pediatrics and of community and family medicine at Dartmouth Medical School and a pediatrician at Dartmouth-Hitchcock Medical Center. "The areas with better than expected rates of low birth weight may be regions with better reproductive and perinatal services."

Dartmouth Medical School