Kate Middleton Slim, Gorgeous in Custom Dress at First Official Event Post-Baby

Credit: Danny Martindale/WireImage

She's baaaack! Kate Middleton was at Prince William's side for the inaugural Tusk Conservation Awards at the Royal Society in London on the evening of Thursday, Sept. 12 -- and for her very first formal, official event since giving birth to son Prince George less than two months ago, the Duchess of Cambridge did not disappoint. Arriving with a tuxedo-clad William at 7:10 p.m., Kate, 31, wore a drop-waist, floor-length, pale gold sequin gown by (who else?) Jenny Packham. The design is from the designer's special line for Debenhams department store, sold for approximately $240 in U.S. currency.

William is set to present two awards inside the event, which celebrates oustanding achievement in the field of African conservation. The Duke, 31, plans to redouble his commitment to the cause after announcing earlier on Thursday that he's left the armed forces after nearly eight years of service in the Royal Air Force.

The couple -- along with baby George, born July 22 -- are expected to move to their new residence at Kensington Palace in London soon.

Those closest to the Duchess, meanwhile, have been marveling for weeks at her amazingly slim figure following the Prince's birth. "A few weeks after George was born, you could hardly tell she had just given birth," the source marveled to Us of the 31-year-old Duchess. "Kate's still breastfeeding, and the small weight she gained while pregnant has just melted off. She's not dieting."