Constance Wu Talks About Her Baby Girl's 'Little Blue Butt': 'It's Very Special'

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Constance Wu is enjoying motherhood.

The Golden Globe nominee, 39, raved about her baby girl's "blue butt" when she appeared Friday on The Tonight Show Starring Jimmy Fallon. "I had a beautiful baby girl in August," Wu said. "She's the best, she's the best. She has a full head of hair. She has a blue butt."

"Yes, her butt is the color blue," Wu explained. "There's this thing, I had never heard of it before. But it's called a Mongolian spot. Apparently, it happens in a lot of Asian babies, and my boyfriend and I are both Asian. And it's where your butt is blue for like the first two years of your life, and then it just goes away."

Officially known as congenital melanocytosis, a Mongolian spot is a birthmark that "are flat, blue, or blue-gray. They appear at birth or in the first few weeks of life," according to Mount Sinai.

"Well, I just think it's a very special thing, and it's not something she gets to show off while she's walking down the street," Wu continued. "Not that she's walking yet. But I thought I would just tell you about her little blue butt. It's very special."

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The Solos actress shares her baby girl with boyfriend Ryan Kattner, who's the frontman for the band Man Man. "He actually wrote a song about her blue butt," she said. "So, that will be on his next album."

Wu and Kattner welcomed a baby girl in August, PEOPLE confirmed. The couple had not previously announced that they were expecting, as they mostly keep their personal lives private.

Wu stars in Amazon Prime's Solos, which also features Anne Hathaway, Uzo Aduba, Helen Mirren, Morgan Freeman, Anthony Mackie and Dan Stevens. The anthology series follows character-driven vignettes about the human condition, with each episode made up of one running monologue.

"They're literally solos," Wu explained of the series, which is now available on the streaming platform. "Every episode is just one actor talking, which I guess could be a nightmare to some people, but it was real fun for us."