BATON ROUGE — A hemangioma is a bright red birthmark that shows up in the first or second week of life. It looks like a rubbery bump and is made of extra blood vessels in the skin.

While they're technically tumors, parents shouldn't let that name or their stark appearance worry them.

"They're actually the most common tumor of infancy, happening anywhere from 1 in 20 to 1 in 50 babies. And, so, just a collection of blood vessels that grows in the first part of a baby's life," Mayo Clinic pediatric dermatologist Megha Tollefson says.

They're more common among babies born preterm. They can appear anywhere on the body but commonly involve the head and neck areas.

Another feature that might alarm parents is the rapid rate of growth. Tollefson says hemangiomas will continue to grow until children are about six to nine months old. At that point, they typically stabilize, then go away slowly.

"We say that 90% of hemangiomas resolve — or involute is what we call it — by age four. So it's a pretty slow process," Tollefson says.

Treatment usually isn't needed. However, Tollefson says if the hemangioma interferes with vision, and other functions, has the potential to be cosmetically disfiguring, or is large or growing rapidly, babies should be evaluated for treatment.