Hold on to baby-belly

New mums are being warned for the sake of their health and their babies' against imitating models and movie stars by rapidly losing weight after childbirth.

What's more, excessive exercise injects lactic acid into the breast milk - making it taste bad.

It's safer to get in shape slowly than to copy celebrities who flaunt skinny post-baby bodies only weeks after giving birth, experts say.

"A lot of women believe that once the baby is out, their job is done," said AUT senior lecturer and dietitian Caryn Zinn. "But they need to be very cautious about going on any kind of diet or stripping their calories. What they eat is reflected in their breast milk."

Supermodel Heidi Klum was on the catwalk modelling lingerie for Victoria's Secret two months after giving birth.

And Nicole Kidman was pictured showing off her flat post-baby stomach two weeks after having a baby last year.

But celebrities who trim down dramatically after pregnancy often do so with the help of personal trainers, nutritionists and chefs.

Zinn said celebrity diets that advocated cutting out carbohydrates were particularly risky.

"It's not a safe thing to do. When you're breastfeeding your requirement for breads and cereals increases."

Most women lose weight naturally while breastfeeding, but should expect to shed less than a kilo per month, she said.

Regular mums shouldn't expect to lose their baby-belly until nine to 12 months after birth, according to Ministry of Health guidelines.

Plunket clinical adviser Alison Jamison said, "Lots of exercise can actually change the taste of breast milk. You get a release of lactic acid in the milk, and some babies don't like it."

She said mums were best to stick with a sensible diet and regular gentle exercise.