Actress Jennifer Aniston is pregnant with triplets - for a hilarious new water advertisement with U.S. comedian Jimmy Kimmel.

The satirical commercial for Glaceau Smartwater, introduced by American Idol host Ryan Seacrest as a news piece about leaked home security from Aniston's house, features the actress taking off a stomach-flattening pad to reveal her fake bulging belly.

Lifting her shirt to expose her baby bump, she asks her unborn kids, "How are my little triplets doing?".

Kimmel makes an appearance as Aniston's grown-up son in a scene around a backyard swimming pool, which is being filled with bottles of Smartwater, and the film star is later seen revealing the secret to her picture-perfect hair as she removes her 'wig' to show her 'real' dark locks - tight curls bunched up in pigtails.

The advert, in which the pampered star also appears to crash a car and hires trainers to lift her up and down during work-out sessions, was designed to poke fun at persistent tabloid rumours suggesting Aniston, who is engaged to Justin Theroux, is pregnant. She has been a spokesperson for Smartwater since 2007.