Dressed in a vibrant green frock, Pippa was positively glowing as she joined in the festivities though she was famously upstaged by her cheeky nephew, Prince Louis.
Pippa and James are already parents to son Arthur, born in October 2018 and daughter Grace, who was born in March 2021.
The new baby is Carole and Michael Middleton's sixth grandchild – along with Arthur and Grace, they are also the proud grandparents to Prince George, who turns nine this month, Princess Charlotte, seven and four-year-old Prince Louis.
Pippa and hedge fund manager James married in 2017 at St Mark's Church in Englefield. The bride wore a stunning lace gown by Giles Deacon and the wedding was attended by the Duke and Duchess of Cambridge and their two eldest children, Roger and Mirka Federer as well as Prince Harry and his then-girlfriend, Meghan Markle.
Pippa rose to fame at her sister's 2011 royal wedding when her fitted bridesmaid dress caught the attention of admirers around the world.
However, despite the fame and pressures of stardom, Pippa revealed in 2014 that she and her sister have "a very normal, sisterly relationship".
"We're very close. And, you know, we support each other and get each other's opinions and things," she said.
"I think that's really the heart for all of us is having a really close family that we can sort of be normal with each other, treat each other normally."
This article first appeared on our sister site, Now To Love.