Tesla CEO Elon Musk and singer Grimes have secretly welcomed a second baby together via surrogate.

Their baby girl's name is Exa Dark Siderael Music, or "Y" for short.

In an interview with Vanity Fair, Grimes revealed that she and Musk secretly welcomed a second child together via surrogacy in December. She revealed the news after the baby began to cry from another room during the interview.

Vanity Fair writer Devin Gordon details the interaction in his feature, saying that he heard something happening in the background of their video call that sounded "vaguely like a lone cry from an infant upstairs."

Grimes had already said that her son, X, was with his dad.

"I think I notice Grimes wince, but I say nothing and move on," Gordon writes, as the cries continue.

"I can tell by the frozen look on my host's face that she heard it too," Gordon continues. "So I brace myself to ask the strangest question of my career."

"Do you have another baby in your life, Grimes?" he asks the pop star.

Gordon writes that Grimes looked away before responding, "I'm not at liberty to speak on these things."

"Whatever is going on with family stuff, I just feel like kids need to stay out of it, and X is just out there. I mean, I think E is really seeing him as a protégé and bringing him to everything and stuff," she says.

After a bit of sidestep rambling, Grimes reveals the truth.

"She's a little colicky too," Grimes tells the journalist. "I don't know. I don't know what I was thinking."

The singer confirms the baby was born in December via surrogate, a few months after she and Musk had ended their three-year relationship.

The family of four is currently living in Austin; however, they have separate homes and separate lives but remain close.

"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," Grimes explains.

"This is the best it's ever been," she continues. "We just need to be free."

In the feature Grimes also shared that she and Musk hope to have more children, telling Vanity Fair that they have "always wanted at least three or four" children together.

Musk shares five sons with his ex-wife, Justine Wilson: twins Griffin and Xavier, 17, and triplets Damian, Saxon, and Kai, 16.

The billionaire was most recently rumored to be dating Australian actress Natasha Bassett after the duo were spotted landing in L.A. and exiting his private jet.

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