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Such defects include being born with a single kidney, having kidney swelling or enlarged ureters, the urine-carrying tubes, said lead researcher Dr. Ian Macumber, a pediatric nephrology fellow at Seattle Children's Hospital.
"The more we find out about obesity, the more we find out what a public health problem it is," he said.
The discovery of a link with kidney and urinary tract birth defects is newer. "We found a significant association between maternal obesity and risk of these anomalies," said Macumber.
While the study found a link between maternal obesity and certain birth defects, it did not establish a cause-and-effect relationship.
The kinds of defects Macumber and his colleagues evaluated are diagnosed in up to 1 percent of pregnancies, with some conditions more serious than others. The defects account for about 20 percent or 30 percent of all prenatal abnormalities, he said.
And risk of these defects increased along with the level of obesity, the study found.
More than half of pregnant women in the United States are overweight or obese, and 8 percent of reproductive-aged women are extremely obese, says the American College of Obstetricians and Gynecologists.
Macumber is to present the findings Nov. 14 at a meeting of the American Society of Nephrology in Philadelphia. Studies presented at medical meetings are typically viewed as preliminary until published in a peer-reviewed journal.
For the study, the researchers looked at medical charts, including birth and hospital discharge records, from 2003 to 2012. They found more than 3,200 cases of abnormal kidney or urinary tract problems and compared them with more than 13,000 "controls" -- newborns without the abnormalities.
Looking at pre-pregnancy weight records, the researchers found that mothers who delivered children with the birth defects were almost 1.3 times more likely to be obese than those whose infants didn't have the kidney and urinary tract defects.
Obesity was based on body-mass index (BMI), a calculation based on height and weight. A BMI of 30 or greater is viewed as obese. As an example, a 5-foot-4-inch-tall woman who weighs 180 pounds has a BMI of 30.9 and is considered obese.
Because the researchers went by hospital codes, they couldn't determine which defects were most common or how severe they were.
The mechanism behind the association isn't known, Macumber said.
"There is certainly some question as to whether insulin may play a role in this," he said. Pregnant women who are overweight may have insulin resistance, a condition in which the body doesn't respond well to the hormone insulin, and blood sugar is less able to get to the cells, he said.
Another expert praised the research.
"Studies like this are really quite important," said Dr. David Mendez, an attending physician at the Miami Children's Hospital and a neonatologist. However, whether obesity actually causes these birth defects is not yet known, he added.
Mendez said that chart reading, while limited in some respects, "is a vital research tool" and a good starting point.
Ideally, future research would follow mothers during pregnancy, tracking their weight and the health of their newborns, looking at the link from the start, he said.
Meanwhile, Mendez advises a good diet, exercise if approved by the woman's doctor and other good health habits. "The things that make you feel good," he tells patients, "will make the baby feel good."
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