The couple, who have been romantically linked since 2012, are said to have arrived alone at Cedars-Sinai Medical Center around 6 a.m. Tuesday and were taken straight to the delivery room, TMZ reported.
Reps for Kunis, 31, and Kutcher, 36, did not respond to requests for comment.
It's the first child for both actors -- though Kutcher did have a father-like role while he was married to actress Demi Moore, who had three kids with her ex, Bruce Willis. Kutcher and Moore finalized their divorce in November 2013.
Kunis' pregnancy reports first surfaced in March, shortly after news that "Two and a Half Men" star Kutcher had popped the question came up in February. The "Ted" and "Jupiter Ascending" actress confirmed she was with child in May during an appearance on "The Ellen DeGeneres Show."
At the time, she told the daytime host that she planned to have a natural birth without the aid of an epidural.
"I'm going to do it as all-natural as I possibly can, unless there's an emergency or something that should go wrong. ... I did this to myself, I might as well just do it right," Kunis said, earning laughs from the audience. "I wanted this," she clarified. "It's not like -- no one made me do this."
She also said that she and Kutcher already knew the baby's gender and had decided on a name, but they would be keeping those details private.
Things she wasn't keeping private? The graphic nature of the birthing process. The "Family Guy" voice went all-in with Marie Claire when describing those plans.
"Two people are allowed in my delivery room," the 30-year-old said. "My doctor and my significant other. And he is staying above the action. He'll be head to head. ... Unless he wants to risk his life and see. But I wouldn't if I were him. I highly doubt he wants to see that being ripped apart and shredded. Because it will be shredded. It's just a matter of how badly."
Mini-Mila is here! Follow me on Twitter @NardineSaad.