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The birthday girl, who welcomed her first child, a baby girl named Khai, with the former One Direction member in September, showed off her incredible post-baby body in a groovy '70s-inspired two-piece ensemble, which included a backless top and a pair of flared trousers.
The supermodel's younger sister also celebrated in style wearing a patterned semi-sheer top with orange boot-cut corduroy pants and square toe boots. She also wore a coordinating brown face mask amid the ongoing COVID-19 pandemic.
The small outdoor birthday gathering included a Gorilla Cheese food truck where attendees snacked on grilled cheese sandwiches.
Loving the Hadid sisters' looks? Recreate Gigi and Bella's outfits head-to-toe with similar styles below.
PrettyLittleThing's Grey Brushed Rib High Neck Flare Jumpsuit retails for $27.
Soda's Western Stitched Ankle Boots retail for $32.99.
SHEIN's Tie Dye Seam Front Crop Top retails for $7.
Glam and Gloria's Rust Colored Velvet Corduroy Flare Pants retail for $49.90.
Kathemoi's Ankle Boots retail for $43.98.