The University of Utah researchers found gastroschisis -- a severe birth defect in which infants are born with their intestines and other organs outside the abdomen -- is four times more likely in babies born to mothers having both infections just before or during early pregnancy.

The study, published in the British Medical Journal, notes the causes and mechanisms of gastroschisis are not yet known, but the age of a woman giving birth appears to play a strong role. Women less than 20 years old are 11 times more likely than women over 25 to have babies with gastroschisis. When the mother is less than 20, the prevalence of gastroschisis is estimated at about one in 570 births in Utah, said study lead investigator Marcia Feldkamp, also of the Utah Birth Defect Network.

"If teens are having sex and getting pregnant, they're at risk of sexually transmitted diseases," Feldkamp said in a statement. "They're not thinking about the consequences, so that's a huge problem with this age group."

Women who have STD often are unaware they have it, Feldkamp said.