The opera singer dazzled at a party in Los Angeles to celebrate the 10th anniversary of Dancing With The Stars, the US version of Strictly Come Dancing.
Katherine competed on the show back in 2012, finishing in second place.
The starlet announced her pregnancy earlier this month, revealing via Instagram that she and filmmaker husband Andrew Levitas are expecting a baby girl.
Katherine broke the news by posting a picture of tiny shoes placed over the couple's wedding photo. The caption read: "Couldn't be more excited to meet our little girl!"
She married American-born Andrew at Hampton Court Palace last year.
Check out some of the best maternity fashion below: