It's a race to the finish line between Rachel Bilson and fellow pregnant brunette actress Mila Kunis ... who'll get to the maternity ward first is anyone's guess!

We spotted the former O.C. star out and about in Hollywood, fueling up with a fruit smoothie. Baby daddy Hayden Christensen wasn't with the mommy-to-be but the Star Wars actor is famously private.

We hope these two decide to show off their little one with a cute Instagram after the birth. That little tyke's gonna be adorable with his parents' good genes!