Elon Musk, who quietly fathered the children with Shivon Zilis, who works for Musk at the company that hopes to develop an implantable computer chip for the human brain, according to documents obtained by Insider, who posted a tweet on May 24 saying “USA birth rate has been below min sustainable levels for 50 years.”

The outlet obtained court documents pertaining to changing the children’s legal names to include Musk’s surname and Zilis’ surname as part of the middle names. CNN Business was unable to independently confirm the contents of the documents, but according to a Travis County, Texas court docket obtained by CNN Business.

Musk tweeted. “Doing my best to help the underpopulation crisis,” he tweeted. “A collapsing birth rate is the biggest danger civilization faces by far.” “Mark my words, they are sadly true” and “I hope you have big families and congrats to those who already do!”

The twins were born in Austin, Texas, according to court documents. According to the documents, Musk and Zilis asked a Texas county court in April to change their babies’ names so they would “have their father’s last name and contain their mother’s last name as part of their middle name.”