The proportion of babies born with a defect called gastroschisis has nearly doubled since 1995, according to a large new study.
The cause of gastroschisis - which is a hole in the baby's abdomen - is unclear, although it's known to be more likely in the children of teen mothers. A mother's exposure to certain chemicals has also been tied to the malformation, though not conclusively.
In the new study, researchers looked at rates of gastroschisis in millions of live births over an 11-year period in the U.S.
"We have a pattern where the prevalence is very much highest among young women and it's growing more rapidly among that group than any other group," said Russell Kirby, a professor at the University of South Florida and the lead author of the study.
Kirby's study could not explain why the birth defect is becoming more common, and gastroschisis itself is not well understood.
The malformation involves an opening next to the belly button, through which the baby's intestines protrude.
Newborns with gastroschisis require immediate surgery to close the hole and put the organs back in place.
Most babies with gastroschisis survive, but Kirby said some children have problems with growth and development and there is not a lot of research about the long term outcomes for these kids.
By general estimates, the condition is relatively rare, with a rate of 2 to 3 cases per 10,000 live births in the U.S. But in recent years, studies have suggested the defect is being seen more often.
To get a better sense of how the numbers have changed over time, Kirby and his colleagues gathered birth defect monitoring data from 15 states.
They found that among 13.2 million births between 1995 and 2005, there were 4,713 babies born with gastroschisis, which translates to about 3.5 out of every 10,000 babies.
This number grew steadily over the study period, however, starting out at 2.3 out of every 10,000 babies in 1995 and climbing to 4.4 out of every 10,000 infants in 2005.
"I do believe the numbers," said James Robbins, a professor who studies birth defects at the University of Arkansas for Medical Sciences.
The increase in gastroschisis primarily affected mothers under age 25, and especially under age 20, whereas those who gave birth in their 30s had no change in their risk of having a baby with the birth defect.
Mothers who had their babies in their early twenties experienced a 5.8 percent increase each year in the risk of having a child born with gastroschisis, Kirby's group reported in the medical journal Obstetrics & Gynecology.
Among these mothers, the number of babies born with gastroschisis went from 4 out of every 10,000 babies in 1995 to 7 in 10,000 babies in 2005.
Teen mothers saw a 6.8 percent yearly increase in the proportion of babies born with gastroschisis.
In 1995, there were 8 babies with gastroschisis out of every 10,000 babies born to women under age 20. By 2005, that number was 15 out of every 10,000 babies.
The proportion of babies with gastroschisis born to Asian women and Native American women remained steady over the study period.
White, black and Hispanic mothers, however, experienced a roughly four to six percent increased risk each year of having a baby with the malformation.
Researchers have not identified what's behind these increases.
A previous study of women in Washington state found that exposure to the weed killer atrazine was tied to an increased risk of having a baby with gastroschisis, although it did not show that the chemical caused the malformation.
Robbins said that smoking is a considered a risk factor, but he doesn't think that's behind the pattern detected in this study.
"I don't think there's a clear explanation for why the rates are going up," said Robbins, who was not involved in the current research.
Kirby speculated that it's possible nutrition could have something to do with the trend, but more research is needed to figure it out.
"We know that there are influences of different vitamins and nutrients that definitely affect fetal development," he said.
But as far as their relation to gastroschisis, "that's just a suspicion."
Kirby's research team is currently following up to see how the numbers have changed since 2005.
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