The two are parents to sons X Æ A-Xii, 3, and Techno Mechanicus, and daughter Exa Dark Sideræl, 22 months

Grimes is opening up about a frustrating moment she experienced with Elon Musk.

In a new biography on Musk, writer Walter Isaacson discusses the SpaceX founder’s 11 kids, including his three children with singer Grimes (real name is Claire Boucher).

In a section about the pair’s son X Æ A-Xii, whom they call “X,” Isaacson reveals that Musk, 52, snapped a photo of Grimes, 35, as she was having a C-section and sent it around to their friends and family, including the musician’s father and brothers.

“He was just clueless about why I’d be upset,” Grimes told Isaacson.

Elsewhere in the book, Isaacson revealed that Grimes and Musk had secretly welcomed a third baby together, a son named Techno Mechanicus, who goes by “Tau." Representatives for both Musk and Grimes did not immediately respond to PEOPLE’s request for comment.

The two already share son X and daughter Exa Dark Sideræl, 22 months. In a March 2022 Vanity Fair interview that revealed Exa Dark’s birth, Grimes admitted that the former couple wanted more babies, telling the outlet, “We’ve always wanted at least three or four.”

But the day the article was released, the “Genesis” singer tweeted that the couple had broken up again.

Along with the three kids he shares with Grimes, Musk is also a father to the six children he shares with his ex-wife, Canadian author Justine Wilson. He is also dad to 22-month-old twins Strider and Azure with Neuralink exec Shivon Zilis.

Grimes previously opened up about co-parenting with Musk in an interview with Wired, saying that the two were in the process of figuring out what their children’s educational journeys would look like.

“I really care about having a very good relationship with my kids. I think I understand how to be a good parent to them. Both enforcing discipline and being their friend,” she said. “Who knows, maybe they’ll resent me and reject family culture, but I feel like they will not.”

In the same interview, Grimes revealed that her son X is following in his dad’s engineer footsteps. When asked if the tech entrepreneur is training their son to be Musk’s “protégé,” Grimes said she was “here for that.”

“X knows a lot about rockets. It’s crazy,” she shared. “He knows more about rockets than me.”

The little one has trouble finding toys he enjoys because, “if they’re not anatomically correct, he gets upset. He’s a little engineer for sure.”