Small and flat. Large and puffy. Bumpy and hairy. Nipples come in all different shapes and sizes. Some are even inverted — they pull inward into the breast rather than pointing outward. And though cursory glances at porn or celebrity nip slips may have you thinking that inverted nipples are weird in some way, don’t worry — in most cases, they’re completely normal. Whether you’ve got a pair of your own, are asking for a friend, or you’re just curious, here’s what you should know about inverted nipples.

You’re usually born with them
If you have inverted nipples, chances are you’ve had them since birth or they inverted during puberty, and there’s no cause for concern. But sometimes they occur after trauma (like an infected piercing) or disease — basically any condition that causes inflammation or scarring of the tissue behind the nipple can cause nipples to invert. “If the nipple suddenly becomes inverted this is a red flag and needs to be addressed by a breast surgeon ASAP,” says Leigh Anne O’Connor, a board-certified lactation consultant in New York City. “If the nipple is inverted since childhood or adolescence it is best to leave it alone — this is only a cosmetic issue.”

There are a bunch of different types
Just like there are different types of outward-pointing nipples, there are different types of inverted ones too. They’re classified by grade: grade 1 inverted nipples can look flat or slightly inverted and become everted when stimulated. Grade 2 have a more inverted appearance than grade 1, but they’re quicker to pop back in after stimulation. Grade 3 won’t become everted even with stimulation. Sometimes both nipples are inverted, and sometimes just one is.

They’re pretty common
About 10 percent of women have inverted nipples, making them more common than red hair (1-2 percent of the population), blue eyes (8 percent of the population), and curly hair (less than 9 percent of women).

Pregnancy might change them
Many parents find that they experience some inversion while pregnant, even if their nipples weren’t inverted before. But by the time they give birth, the skin has become more elastic, at which point even those who’ve always had inverted nipples end up with protruding ones. And in most cases, a nursing baby will pull out the nipple, O’Connor says.

There are treatments available, but you probably don’t need them

From suction devices to nipple stimulation to surgery, there are plenty of ways to stretch the tissue and draw out an inverted nipple, if it bothers you. But if your inverted nipples weren’t caused by disease or trauma, there’s no reason you need to mess with them — and if you do, O’Connor cautions that you might want to wait until you’re done having kids. “Surgically changing it can make breastfeeding challenging in the future,” she says. “Many young women do not consider breastfeeding in their future and I have seen many new moms regret cosmetic surgery when it interferes with feeding their baby.”

Bottom line
For the most part, you can think of nipples like bellybuttons. Some are innies, some are outies, and all are equally awesome. But if you’re post-puberty and you’ve noticed a change in your nipples — like if one or both of your nipples have only recently become inverted — see your healthcare provider to be safe.

Diana Vilibert is a freelance writer and copywriter living in Brooklyn, NY. She loves flea markets, martinis, to-do lists, traveling, and wearing leggings as pants. You can see more of her writing at www.dianavilibert.com and follow her on Twitter at @dianavilibert.