What Is Congenital Dermal Melanocytosis? Constance Wu Says Daughter Has a Blue Birthmark on Her Butt

Crazy Rich Asians star Constance Wu lives a pretty private life off-camera, but she opened up about her 9-month-old daughter during a recent interview. One tidbit she slid in there: Her baby has a blue butt.

Wu, 39, recently went on The Tonight Show Starring Jimmy Fallon and gushed about her daughter. "She's the best! She has a full head of hair, she has a blue butt," she said. Fallon started to ask about whether Wu's daughter is sleeping well, and Wu interjected. "You didn't ask me about the blue butt, Jimmy! I thought that you might be curious about the baby with the blue butt," she said.

Fallon admitted that he "didn't understand" what Wu was talking about so he "kinda skipped over it." Finally, he asked about Wu's daughter's blue butt.

"Yes, her butt is the color blue," Wu said. "There's this thing, I had never heard of it before, but it's called a 'Mongolian spot.' And apparently, it happens in a lot of Asian babies, and my boyfriend and I are both Asian."

The condition, Wu continued, is "where your butt is blue for the first two years of your life and then it just goes away."

Wu said that her daughter's blue butt is a "very special thing," joking that it's "not something that she gets to show off while she's walking down the street—not that she's walking yet."

Worth noting: The medical community is veering away from using the term "Mongolian spot" due to its racist undertones, but there is a proper term for the condition—congenital dermal melanocytosis. And yep, this is a thing.

OK, so why would a baby have a 'blue butt'?

According to the American Academy of Dermatology (AAD), congenital dermal melanocytosis usually looks like a bruise around the butt. It can be light blue, dark blue, or blue-gray, and a baby can have one spot or several. Each spot can "vary greatly" in how big it is, the AAD says.

Congenital dermal melanocytosis has a few different symptoms, according to Mount Sinai, including:

• Blue or blue-gray spots on the back, butt, base of spine, shoulders, or other areas of the body
• Spots that are flat with an irregular shape and unclear edges
• Spots that have a normal skin texture
• Spots that are two to eight centimeters wide or larger

What causes a 'blue butt,' and who usually gets these birthmarks?

The unique color of congenital dermal melanocytosis is from a collection of melanocytes—cells that make the color of the skin—in the deeper layers of the skin, according to Mount Sinai.

While the AAD says that congenital dermal melanocytosis is most common in people of Asian descent, it happens in all races. However, it's least common in people with white skin.

Most cases of congenital dermal melanocytosis appear on a child's lower back or butt, but they can technically show up anywhere on the skin, the AAD says. And, Wu is right, this birthmark usually goes away over time, with the AAD saying that it usually disappears by the time is a child is three to five years old.