The couple, who starred on the sitcom "That '70s Show," where they played the onscreen couple Jackie and Kelso, became engaged in February.
Mila, 30, told Marie Claire that "the best day of my life so far" was when Ashton, 36, proposed to her. This will be Mila's first marriage and the second for Ashton, whose six-year marriage to Demi Moore ended in December of 2012.
The lovebirds have been photographed throughout the "Jupiter Ascending" actress's pregnancy packing on the sweet PDA, while out and about on a romantic movie date, at Dodgers baseball games, and during a frozen yogurt run.
In July, the "Two and a Half Men" actor's twin brother Michael said he thought the two were "meant to be. It'd be like Ross and Rachel getting married [from 'Friends']. They make such a great couple."
Gushing to W magazine in August, Mila said she "never wanted to get married … then things changed -- I found the love of my life. We all get movie star crushes. I'm marrying mine."
The "Family Guy" star revealed to Ellen on "The Ellen DeGeneres Show" in May that Ashton had stocked up on foods he thought Mila would crave during the pregnancy.
"[Ashton] assumed that I was gonna have goofy cravings, so he stocked our secondary fridge with weird food -- just, like, pickles and sauerkraut, or, like anchovies and ice creams -- just in case at one point during this pregnancy I'd be like, 'I really want something,'" she said. Aww!
Ashton also hit the books and studied Russian -- Mila's family is from Ukraine -- so he could learn to speak the language before the baby came.
The couple sold off their separate homes -- Mila put her Los Angeles mansion on the market in August and Ashton soon followed, selling his Hollywood Hills bachelor pad for almost $10 million. The two bought a home together that same month.
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