News broke that the actress was expecting her second child last month (Jul21), but neither Scarlett nor comedian Jost commented at the time.
But the "Saturday Night Live" star reportedly made the big reveal during a stand-up show at the Ridgefield Playhouse in Connecticut over the weekend (14-15Aug21), telling the audience, "We're having a baby, it's exciting."
No other details have been released.
This is Colin's first child while Scarlett has a six-year-old daughter, named Rose, from her marriage to French journalist Romain Dauriac.
Last month a source told Page Six, "Scarlett is pregnant but has been keeping it very quiet. She has been keeping a very low profile."
"She hasn't been doing many interviews or events to promote Black Widow, which is surprising since it is a huge Marvel/Disney release and she is both the star and an executive producer."
Instead, she carried out promotional appearances via Zoom.
She was also absent from a "Black Widow" screening in the Hamptons, New York on 2 July (21).
Scarlett Johansson and Colin Josh married in October 2020 after more than a year of engagement. They kept the celebration low-key due to pandemic.
Although it was a small affair, the actress admitted she felt "a little stressful" because there were so many elderly relatives involved. "Obviously we wanted to take all the precautions that we could, so we had the (face) masks... and everybody had to get (COVID) tests like, 100,000 times," she explained.
"It was beautiful and we were so happy to see people that we hadn't seen in such a long time," she added. "We just felt very fortunate to be able to get together like that. It felt like it was sort of how it was supposed to be."