52-year-old Hilary Swank's got a bun in the oven ... according to the announcement she made online.

The actress took to social media to post a video of herself, her husband, and their twins dancing in the dark dressed as glow-in-the-dark stick figures.

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When Hilary turned around, she revealed a stick figure baby in her stomach ... signaling she and Philip Schneider are expecting their third kid.

She captioned the cute clip ... "A lot of people have been asking what project I’m doing next, this one releases in December."

The "Million Dollar Baby" star gave birth to her twins, Aya and Ohm, just 3 years ago ... and it looks like she's gearing up for baby number 3!