Grimes, left, and Elon MUSK attend The Metropolitan Museum of Art's Costume Institute benefit gala in New York. AP

Grimes has revealed, seemingly accidentally, that she has welcomed her second child with Elon Musk.

In a new interview, the musician invited writer Devin Gordon to her home in Austin, Texas.

When the journalist heard a baby cry, they asked if Grimes had another child apart from the one-year-old son, named X Æ A-12, she shares with the tech billionaire.

“I’m not at liberty to speak on these things,” Grimes, real name Claire Elise Boucher, told Vanity Fair in response. “Whatever is going on with family stuff, I just feel like kids need to stay out of it, and X is just out there.

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“I mean, I think E is really seeing him as a protégé and bringing him to everything and stuff... X is out there. His situation is like that. But, yeah, I don’t know.”

She then admitted that she and Musk recently welcomed a baby girl, Exa Dark Sideræl Musk, who was born via a surrogate. The couple have nicknamed the baby “Y”.

Grimes said her new baby girl is “colicky” and so was more likely than usual to be crying. “I don’t know what I was thinking,” she said of her decision to invite a journalist to her house.

It had been reported in September last year that Grimes and Musk had split up, but speaking about their relationship, Grimes told the publication: “There’s no real word for it. I would probably refer to him as my boyfriend, but we’re very fluid.”

She added: “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...

“This is the best it’s ever been…. We just need to be free…We’ve always wanted at least three or four [children].”

Following reports of her split from Musk, Grimes was photographed in October reading Karl Marx’s The Communist Manifesto.

Responding to fans who joked that she had returned to her “communist roots”, the artist said she had been “trolling” people with the pictures after becoming “really stressed” by photographers following her.

“Paparazzi followed me to a shoot so I tried to think what I could do that would yield the most Onion-ish possible headline and it worked,” she tweeted at the time.