Mila is said to have given birth to their daughter with Ashton by her side in Los Angeles on Tuesday.
The couple's daughter is their first child together and her reported arrival comes days after they were spotted walking their dogs in California.
Reveal has contacted Mila and Ashton's reps for a comment.
Mila, 31, and 36-year-old Ashton met on the set of That 70s Show in the 90s, however they weren't linked romantically until early 2011.
Ashton had previously been married to fellow Hollywood star Demi Moore but they split after six years.
In February this year, Ashton and Mila got engaged after he popped the question with a dazzling diamond and they announced Mila's pregnancy a month later.
Celebrities who have welcomed babies in 2014:
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