She gave birth to son Bingham just two months ago, but that didn't stop Kate Hudson from stepping out in a super-slinky dress Tuesday night.

PHOTOS: Kate Hudson's bohemian-chic pregnancy

The Something Borrowed star -- who a source tells Us Weekly "didn't gain a lot [of weight] this time around" -- arrived with her fiancee Matt Bellamy, 33, at pal Stella McCartney's 40th birthday celebration in London wearing a coral jersey gown with a draped plunging back and thigh-high slit by Elie Saab.

Hudson, 32, accessorized with jeweled platform pumps and piled her blonde hair into a loose updo adorned with flowers.

PHOTOS: Kate's bikini bod over the years

Also on hand to celebrate with the British designer was Kate Moss, Gwyneth Paltrow, Bono, Tom Ford and Stella's father Sir Paul McCartney, who will marry love Nancy Shevell this weekend, a source confirms to Us.

Get more Us! Follow us on Twitter, Friend us on Facebook, Subscribe to Us Weekly