Unless it's advertising diapers, maternity clothes or food, some stars probably wouldn't even dream of taking part in an advertising campaign while heavily pregnant.
But Sienna Miller wasn't going to let her huge bump stop her from working - especially when she didn't have to hide it from the cameras.
The actress was snapped proudly showing off her burgeoning stomach as she filmed an ad in London yesterday.
Working mum-to-be: A heavily pregnant Sienna Miller was snapped in north London yesterday in skinny jeans and a black shrug as she filmed an advert for a cosmetics company
It was all too clear to see through her blue and white striped shirt as she filmed on a street in Primrose Hill, north London.
Sienna, 30, was snapped in skinny blue jeans, brown ankle boots and a shaggy black cape as she walked along the street near Regent's Park clutching a designer bag.
She was filming the ad for Amorepacific, a Korea-based company for beauty and skincare products.
Sienna is expecting her first child with boyfriend Tom Sturridge, who she's been dating for over a year.
This week she spoke to Vogue magazine for its April addition and spoke about being pregnant.
She said: 'I'm feeling fine. It's all progressing nicely and it's very exciting.
'I feel very grounded, more than ever - it's a wonderful feeling. But, all of a sudden, I've started to get cravings. I usually don't have a sweet tooth but these tarts I could eat all day long!'
Sienna said she realised the irony of talking about her impending motherhood especially after her fight to maintain privacy and her legal action against the now-defunct News of the World and giving evidence at the Leveson Inquiry.
She added: 'I'm in my second trimester, so I'm I-can-talk-about-it pregnant, even though I'm under strict instructions not to say anything as it would completely defeat the point of everything I've tried to achieve in the last eight years.'
