Aussie film star Isla Fisher, the fiance of comedian Sacha Baron Cohen, has confirmed she's pregnant.

The former Home and Away actress admitted she had initially tried to disguise her pregnancy, claiming she didn't want to talk about it until she was sure there were no problems.

She told Movies.com: "In the beginning I dodged the rumours...I was trying not to be rude, but there's a time before you're sure everything is all right with no complications that you just don't want to discuss it."

The Wedding Crashers star joked that she wasn't sure if the baby she was carrying belonged to her current partner Baron Cohen, saying: "I'm doing DNA tests with the cast of Hot Rod and the cast of Wedding Crashers, and hopefully I'll find out for sure."