For every 1.1-pound decrease in birth weight, the risk of the birthmark increased nine-fold, Beth Drolet, M.D., of the Medical College of Wisconsin, and colleagues reported in the November issue of the Journal of Pediatrics.
"Based on low birth weight statistics, we estimate that the incidence of infantile hemangiomas has increased by 40% in the last 20 years," Dr. Drolet said.
According to the researchers, there's been an increase in the number of U.S. infants who are born at less than 5.5 pounds. The 8.2% below that weight in 2005 was the highest percentage since 1968, and a rate higher than in most industrialized countries, the researchers said.
Dr. Drolet said her institution has also seen a "dramatic increase" in the number of babies with hemangiomas, a common birthmark caused by a conglomeration of blood vessels.
Previous studies have found that other risk factors for hemangiomas include premature birth, female sex, white race, increased maternal age, maternal history of infertility, using assisted reproductive technologies, and having a prior miscarriage.
But the results of this study, Dr. Drolet said, might explain why more infants are developing hemangiomas.
"This study reaffirms several known risk factors for infantile hemangiomas, … but for the first time demonstrates that low birth weight rather than prematurity per se is the most significant risk factor for developing hemangiomas," the authors said.
Their case-control study compared 420 children who had been diagnosed with a hemangioma at the Children's Hospital of Wisconsin and the University of California San Francisco Medical Center with 353 children under age two who had been diagnosed with skin anomalies other than the birthmark.
Infants with hemangiomas were more likely to be female (P<0.0001), non-Hispanic white (P<0.0001), premature (P<0.0001), and of low birth weight (P<0.0001).
Compared with the control group, the odds ratio for hemangiomas in preterm infants was 1.53 (95% CI 0.86 to 2.7) for birth weight below 2,500 g, and the odds ratio was 2.8 (95% CI 1.46 to 5.6).
In the control group, 4.8% of infants were the product of multiple gestation, compared with 10% of infants with hemangiomas (P=0.0064). Of controls, 2.8% were the product of in vitro fertilization compared with 4.2% of the hemangioma patients (P=0.336).
The multivariate analysis also showed that 33% of infants with hemangiomas had a positive family history of the birthmark compared with 15% of the control group (P<0.001).
"The finding that a significantly higher percentage of children with infantile hemangiomas had a positive family history suggests at least some genetic predisposition," Dr. Drolet said.
Even in the regression model, low birth weight was still the most statistically significant risk factor for hemangiomas, the researchers said.
They called for further research into medications used to treat hemangiomas, because there are no FDA-approved medications to treat the condition.