Elon Musk and one of his top executives secretly had twins in November, according to a report Wednesday by Insider.
The revelation came in court documents petitioning to have the children's last name changed to Musk.
Zilis formerly lived in San Francisco, but she bought a house in Austin about three months before the twins were born, Insider reported.
The heavily redacted name-change petition lists that address for both Musk and Zilis, Insider reported.
Musk tweeted about declining birth rates in 2017, saying "The world's population is accelerating towards collapse, but few seem to notice or care.".
In May, Musk tweeted a Wall Street Journal graphic tracking a "fertility slump" in the US since 1940.
I am a rare exception," Musk tweeted, adding later in the thread, "I mean, I'm doing my part haha. »