Premature babies born in the United States are more than twice as likely to have a major birth defect than full-term infants, with the risk even higher among very pre-term babies, researchers said on Wednesday.
The researchers tracked nearly 7 million babies born between 1995 and 2000 in 13 states, accounting for about 30 percent of U.S. births, to better understand the relationship between birth defects and pre-term birth.
Most pregnancies last roughly 40 weeks. Babies born before 37 weeks of pregnancy are considered premature, and they are at risk for a range of health problems including birth defects.
About 8 percent of babies born prematurely had a birth defect, compared to roughly 3 percent for full-term births, according to the researchers.
Babies born between 24 and 31 weeks gestation were at the highest risk for birth defects — about five times more likely than full-term infants, they said.
The most common birth defects in this group were central nervous system defects such as spina bifida and cardiovascular defects such as a hole in the heart.
Among all pre-term babies, 8 percent had major birth defects. The number jumped to 16 percent of the very pre-term babies — those born between 24 and 31 weeks gestation.
Cause remains elusive
The study was led by Margaret Honein at the U.S. Centers for Disease Control and Prevention's National Center on Birth Defects and Developmental Disabilities.
"Pre-term birth affects about 12 percent of infants born in the United States. And birth defects affect about 3 percent of births in the U.S. And we think it's important to recognize the pretty strong association between these two serious outcomes," Honein said in a telephone interview.
The cause of most birth defects remains elusive, although it is likely the most common birth defects stem from a combination of genetic and environmental factors, Honein said.
She noted that smoking during pregnancy and maternal obesity before pregnancy are risk factors for some major birth defects and for premature birth.
The March of Dimes advocacy group said the percentage of U.S. babies born pre-term has been rising since 1981.