Babies born at a low weight are more likely to get infantile hemangiomas, a common birthmark, according to a new study.

"Hemangiomas are benign tumors composed of blood vessels. Our institution has seen a dramatic increase in the number of infants presenting for care with hemangiomas. We believe the results of this study provide an explanation for this emerging pediatric health issue," said Dr. Beth Drolet of the Medical College of Wisconsin, who led the study.

Being white, a girl or premature have also been found to be causes of birthmarks, but Drolet found that low weight was the most significant risk factor.

"For every 1.1 pound decrease in birth weight, the risk of hemangioma increased by nine-fold," she said.

There has been a trend toward low-weight babies in the U.S., with 8.2 percent born under 5.5 pounds in 2005, the highest rate since 1968.

"Based on low birth weight statistics, we estimate that the incidence of infantile hemangiomas has increased by 40 percent in the last 20 years," Drolet said.

Hemangiomas are not visible at birth, but show up in the first few weeks. They can cause scarring or other medical issues that require treatment, though the FDA has not approved any treatment for hemangiomas themselves.