What causes birthmarks? They're often harmless, but their origins have puzzled doctors for years. More than 10 % of babies are born with some kind of birthmark.

Some fade over time, while others stay for life.

"Some types of birthmarks that we see, have to do with how the skin cells migrate in the skin during development as well," said Dr. David Pearson a dermatologist with M Health Fairview. "It depends on the birthmark."

"So it is forming in utero, in the womb, but not necessarily because of external events. So, you know, maternal diet and things like that probably don't play a major role most of the time," said Pearson.

Pearson says birthmarks can be caused by extra blood vessels which often show up as red. Extra pigment cells usually show up darker.

"Some types of birthmarks we call Cafe Au Lait macules," said Peterson. "And those are kind of like light tan spots that we see like coffee, like coffee exactly, coffee with milk Exactly. So we see those in a lot of the population."

Pearson also says that genetics can play a role — but in general birthmarks aren't inherited.

"Cafe Au Lait macules, the coffee and milk types of spots those people don't grow out of," said Pearson.

If a birthmark affects self-esteem, there are treatment options like topical meds, laser therapy, or surgery.

Pearson also says that most birthmarks don't carry an increased risk of associated skin cancers.

Moles that develop after birth aren't birthmarks — neither are freckles.

Anyone concerned about their skin should consult a doctor.

"We can step in and say, Yeah, this is what it is. Don't worry about it, you know. Or maybe we keep a little bit of an eye on it, but yeah, we see a lot of birthmarks, very common," said Pearson.

If you do notice changes to a birthmark, contact your healthcare provider.