Grimes has branded Elon Musk "clueless" for sending pictures of her undergoing a caesarean section to family members.
The singer - real name Claire Boucher - has three children with the tech billionaire and she's revealed she was furious with her on/off partner when she discovered he had sent her father and brothers intimate snaps of the moment she gave birth to their son X Æ A-12.
In journalist Walter Isaacson's new biography of the Tesla boss - titled 'Elon Musk'- Grimes explained: "He was just clueless about why I'd be upset."
The book also revealed the existence of the couple's third child - a son named Techo Mechanicus who goes by the name Tau.
It was previously thought the pair were parents to two children together, three-year-old X Æ A-12, and daughter Exa Dark Sideræl, 19 months, but Isaacson stated they also have second son Tau.
The revelation was mentioned in an extensive review of the book by the New York Times newspaper.
The outlet wrote: " Isaacson has ably conveyed that Musk doesn’t truly like pushback. Some of his lieutenants insist that he will eventually listen to reason, but Isaacson sees firsthand Musk’s habit of deriding as a saboteur or an idiot anyone who resists him.
"The musician Grimes, the mother of three of Musk’s children (the existence of the third, Techno Mechanicus, nicknamed Tau, has been kept private until now), calls his roiling anger “demon mode” — a mind-set that “causes a lot of chaos."
Musk is believed to have 11 children in total, including his three kids with Grimes.
The singer recently revealed her elder children, who are known as X and Y respectively, share their father's interests in space and engineering.
Speaking to Wired before the ongoing SAG-AFTRA strike, the 35-year-old star said: "X knows a lot about rockets. It’s crazy. He knows more about rockets than me.
“We had to stop giving him toys, because if they’re not anatomically correct, he gets upset.
“He’s a little engineer, for sure. But his obsession with space is bordering on, ‘Is this healthy?’ "[Y is] a little engineer too. She likes industrial shipping. She’s very strange."