Kate announced her pregnancy in December, though until now evidence of it was not obvious, CNN reported Friday.
On Wednesday she worked a cocktail dress designed by Emilia Wickstead that clearly showed off her growing baby bump, People magazine reported.
The dress was adapted from a non-maternity design from the designer's spring/summer collection, People said.
"It was made specially for the duchess," a spokeswoman for the designer said. "We've had a style in store that's very, very similar. So it was very slightly bespoke for her."
The dress came from a collection inspired by Truman Capote's "society swans," People reported.
The dress sells for $1,500, but Kate's alterations would have set her back another $300.
Royal chronicler Judy Wade said she believes the dress to be Kate's first maternity dress.
"Even though it's not designed directly for pregnant women," she said, "it is easy to adapt because it is an empire line design."