The singer shone in a pretty frock designed by Suzanne Neville while performing at British Airways' annual ball in aid of Comic Relief, held at Wentworth Golf Club.
Katherine said: "There's nothing quite like performing in front of an intimate crowd, we all had a great time and the night played host to some very generous individuals.
"I'm honoured to be part of such a special occasion to raise money for Comic Relief!"
All proceeds from the Ball go to Flying Start, the airline's charity partnership with Comic Relief, to help vulnerable children in the UK and abroad.
Katherine, 34, is just back in the UK after a fleeting visit to the US to appear on the Dancing with the Stars anniversary special. She was a contestant on the US version of Strictly Come Dancing in 2012.
The Welsh star announced this month that she and American director/writer husband Andrew Levitas are expecting their first child together, a baby girl
Katherine and Andrew married at Hampton Court Palace last year.
Check out some of the best maternity fashion below: