No wonder she looks so great at 8 months along -- she's been a dedicated gym rat since day one of her pregnancy!

On Friday Mila Kunis went to the gym in Los Angeles, and the 31-year-old actress showed off her giant belly in a black tank and spandex. We haven't seen her out with Ashton Kutcher in a few weeks, but it's a lot easier for him to fly under the radar -- he's not the one trying to hide a baby bump!

Mila hasn't revealed her exact due date, but according to a number of reports she's set to give birth within the next few weeks. Earlier this spring she said she wanted to deliver her baby naturally, but a source recently told OK! Magazine that Mila is starting to worry about that decision.

"Mila wants the world to think she's calm and collected, but in reality, she's freaking out," an insider tells the mag. "She's starting to really panic as the due date gets closer. She's second-guessing all her decisions and is panicking about the pain. She's been reading things online about natural births, and they're scaring her even more. She's beyond fearful."