Grimes attends the Marvel Studios "Captain Marvel" premiere on March 04, 2019 in Hollywood, California. Elon Musk attends TIME Person of the Year on December 13, 2021 in New York City.

Grimes is a mom of two! The singer, 33, secretly welcomed another baby with Elon Musk, she revealed to Vanity Fair in a cover story for the magazine’s April 2022 issue.

Grimes, who shares 22-month-old son X Æ A-12 with the Tesla founder, 50, told the publication that she and Musk welcomed a daughter last December via surrogate. The last month of her pregnancy with X, she couldn’t walk. “He was pressing on my nerves, so I kept collapsing,” she said. “I took a few steps and collapsed. It was kind of scary, because you don’t want to fall a lot when you’re eight months pregnant. So I would just crawl to the bathroom and crawl back or whatever.” At one point during the pregnancy, she thought she was dying. “Like, I hemorrhaged. It was scary.” She and Musk wanted more kids, but she feared serious complications.

The couple used a surrogate this time, which in combination with the pandemic enabled them to keep their daughter, Exa Dark Sideræl Musk, a secret. Grimes explained her daughter’s unique name, with Exa referring to a supercomputing term, exaFLOPS, while Dark represents “the unknown.” She explained, “People fear it but truly it’s the absence of photons. Dark matter is the beautiful mystery of our universe.”

Sideræl is pronounced, as you may have guessed, “sigh-deer-ee-el,” with Grimes calling her spelling of the word as “a more elve true time of the universe, star time, deep space time, not our relative earth time.” Sideræl has a double meaning, for Grimes’ favorite Lord of the Rings character, Galadriel, who “chooses to abdicate the ring.”

In December, Grimes posted an image of herself pregnant on Instagram:

Grimes was “fighting” to name her daughter Odysseus Musk, because “a girl named Odysseus is my dream,” but she and Musk settled on Exa Dark Sideræl instead.

Despite splitting up last September after three years together, Grimes told Vanity Fair that she and Musk are back together. “There’s no real word for it,” she said. “I would probably refer to him as my boyfriend, but we’re very fluid. We live in separate houses. We’re best friends. We see each other all the time…. We just have our own thing going on, and I don’t expect other people to understand it.”

She added that she and Musk are happy together, telling the outlet, “This is the best it’s ever been. … We just need to be free.” She told the outlet that they plan to have more children too: “We’ve always wanted at least three or four.”