Babies who are prematurely born have a two-fold risk of developing major birth defects, in comparison to full-term newborns, according to a study released on Wednesday.

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, as Reuters informs.

Infants born after thirty-seven weeks of pregnancy or less were considered premature births.

About 7 million babies born between 1995 and 2000 were tracked. Approximately 8% of the pre-term babies had a birth defect, in contrast to 3% of those born normally.

Babies born between twenty-four and thirty-one weeks were exposed to a five-fold risk of birth defects than the full-term ones. The most common birth defects were central nervous system defects such as spina bifida and cardiovascular defects such as a hole in the heart.

"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," said Dr. Margaret Honein in an interview with Reuters.

It is not yet clear what the cause of the birth defects is, but it seems a complex of genetic and environmental factors leads to them.