Some things in life are absolutely certain: The sky is blue, Chris Evans is the best Hollywood Chris, and nipples always point outward...right?

Yes, definitely, to the first two—but as for the nipples thing? Maybe not. You see, while most nipples point outward (hello, headlights), it's totally possible for some types to, uh, retreat back into the breast.

Sometimes, it's totally NBD (you were born that way, baby); but other times, your nips might be trying to tell you something big is up—and you'd better listen.

Hold on, what exactly are inverted nipples?

Basically, they're nipples that appear to be retracted into the breast, instead of pointing outward.

Fewer than 5 percent of people are born with inverted nipples, says Stephanie Downs-Canner, M.D., an assistant professor of oncological breast surgery at the University of North Carolina.

This can happen when the lactiferous ducts in your nipples—which are responsible for delivering milk to the surface—are shortened or not fully developed, ultimately pulling back on the nipple. In these cases, the direction of the nipples is not necessarily indicative of anything, says Downs-Canner.

However, if your nipples have always pointed out, and now they've suddenly become inverted, that might be a red flag.

Well, what causes inverted nipples, and should I worry about them?

Again, if your nipples have always been inverted, take a seat, you’re good.

As for everyone else, one of the first things Downs-Canner checks up on when she encounters a newly inverted nipple is the possibility of an underlying cancer. For this reason, she stresses seeing a physician the moment you notice your nipple’s retracted in order to increase your chances of early detection.

But, while a sudden change in your nipples is something to definitely get checked out, your mind shouldn't go straight to a cancer diagnosis: “Other women can have inverted or retracted nipples if they’ve had a lot of infections of their breast,” says Downs-Canner.

Mastitis—inflammation of the breast tissue, primarily caused by breastfeeding—is an example of one of those infections that can cause retracted nipples, says Downs-Canner.

Another benign cause of nipple inversion: ectasia, or a clogged milk duct. “It happens when the ducts of the breast become frequently blocked or clogged," says Downs-Canner. FYI: You can get a clogged milk duct even if you're not breastfeeding. “It can happen to anyone,” at any time, says Downs-Canner.

Well, is there anything I can do about inverted nipples?

Depending on the cause, your nipple might eventually pop out again. After treatment for a breast infection, for example, there's a good chance your nipples will return to their original proud stances, says Downs-Canner.

For those born with inverted nipples, plastic surgery can help make them stick out, says Downs-Canner, though, not a ton of people opt to do this (and it's also not medically necessary): "Most people who are born with inverted nipples typically don’t seek cosmetic surgery," she says.

One more thing: When it comes to breastfeeding with inverted nipples, that's totally doable too, says Downs-Canner—it just might take a little more patience (and the help of a lactation consultant).