On Thursday, federal researchers reported that a fatal birth defect has been increasingly seen in maternity wards, and the cause isn’t known so far.

Infants suffering with the defect, known as gastroschisis, take birth with intestines poking out of a hole in the abdominal wall. Other organs, including the stomach and liver, could also be found outside the body when the baby is born.

The Centers for Disease Control and Prevention said that the prevalence of gastroschisis has seen a roughly 30% rise to 4.9 births out of 10,000 during the years from 2006 to 2012, from 3.6 per 10,000 live births from 1995 to 2005.

The defect is generally suffered by babies born to mothers below 20, though the rate has seen a rise among mothers of all age groups. Young white mothers deliver defected babies more than young women belonging to other races: 18.1 per 10,000 live births, in comparison to 16.1 for Hispanic mothers and 10.2 for black mothers.

However, during the period taken into consideration for study, gastroschisis increased most significantly in babies born to non-Hispanic black mothers with age 20 or below, rising 263%.

Coleen A. Boyle, the director of the C.D.C.’s National Center on Birth Defects and Developmental Disabilities, said, “We don’t know why. We continue to be concerned that this condition is increasing, and we do see a more rapid rise among non-Hispanic black teens”.

If you smoke or drink alcohol, then you are more likely to have a baby with gastroschisis. But Dr. Boyle said that those were risk factors, not causes. And it isn’t known so far whether those risk factors have been contributing to the rise or not.

The report observed that the increase had no relation with the number of babies born to teenage mothers, which has fallen since 1995.