Elon Musk on birth of twins with top exec: 'Doing my best to help the underpopulation crisis'

Elon Musk warns of collapsing birth rate in the U.S.: 'underpopulation crisis'

Elon Musk acknowledged the overlapping pregnancies of two of his partners after the birth of his latest children — a set of twins.

Musk reportedly had the children with Shivon Zilis, 36, an executive at his cognitive technology company Neuralink.

The twins were born in November in Austin, Texas. The newborns arrived just weeks before Musk and Claire Boucher, the musician known as Grimes, had their second child via surrogate in December. 

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"Doing my best to help the underpopulation crisis," Musk said on social media. "A collapsing birth rate is the biggest danger civilization faces by far."

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The Tesla CEO doubled down, saying, "Mark my words, they are sadly true."

"Population of Mars is still zero people!" Musk continued. "I hope you have big families and congrats to those who already do!"

In April, Musk and Zilis filed a petition to give the twins their father's last name and their mother's last name as part of their middle name, according to court documents.

Elon Musk

Tesla CEO Elon Musk (Reuters/Michele Tantussi/File Photo / Reuters Photos)

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A month later, a Texas judge approved the petition, the report added.

Zillis has recently been floated as one of the people Musk could tap to run Twitter once his $44 billion acquisition deal is finalized, according to the report.

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The news brings Musk's total count of children to nine. Musk shares two children with Grimes. He also has five kids with his ex-wife, Canadian author Justine Wilson.

Musk is the wealthiest person in the world, with an estimated net worth of about $220 billion.

FOX Business' Ken Martin contributed to this report.