According to TMZ Mila, 31, and Ashton, 36, were seen arriving at the Cedars-Sinai Medical Centre at 6am and were taken straight into the delivery room.
A source later confirmed the news to US Weekly and said they'd had a "healthy baby girl" and "are so happy!"
"Ashton and Mila feel like they won the baby lottery," a friend told OK! magazine soon after the pregnancy was announced. "It was a total shock for both of them, but they feel up for the challenge.
"It didn't seem like it was possible for them to fall any deeper in love, but the prospect of raising an insta-family together has solidified their commitment to each other."
It sounds like marriage could be just around the corner for the proud parents, too.
"I never wanted to get married... then things changed - I found the love of my life," she said in an interview for its August issue. "We all get movie star crushes. I'm marrying mine." Cute!
Of course, Mila isn't the only Hollywood star to welcome a baby this year. See more celebrity baby bumps below: