The fashion designer has revealed in an interview in The Sun-Herald's Sunday Life magazine that she and her husband underwent several years of IVF treatments, which resulted in much heartache.

''There's a perception out there that so many fabulous Hollywood stars in their 40s are having children,'' she says.

''I have so many friends who are trying to have children and they can't. It's your body's natural clock. You're not meant to have children in your 40s and 50s.''

Dinnigan turns 47 next month, and the baby is due in November.

IVF Australia figures show that the chance of a 46-year-old woman conceiving naturally is just 1 per cent, and conceiving with IVF using her own eggs is also just1 per cent.

The miscarriage rate for women aged 40 and over is 25 per cent; for women aged 45 and over, the rate is 50 per cent.

For Dinnigan, who last year wed hotelier Bradley Cocks, 36, it will be her second child. She became pregnant after the couple decided to switch to what she described as ''a more natural approach''.

But she concedes that pregnancy at her age is very unusual and difficult to achieve. Women had to be ''mindful'' that they ''only have so many eggs and that their quality deteriorates with age''.

''In my 30s, I didn't even realise that women's eggs aged,'' she says.

''Even in your 30s, you think you're invincible. You have a career, life slips by and suddenly you're 40.''

Dinnigan is vegetarian, a non-smoker, eats only organic food, and says she tried a number of combinations of vitamins and exercise to help her conceive naturally, but admits this may have had no effect.

''We might just be very, very lucky,'' she says. ''We are thrilled. You can't expect to have a child at this age. We're blessed.''

IVF Australia's medical director, Associate Professor Peter Illingworth, recommends women over the age of 44 look for an egg donor.

''I see women in their mid-40s who are in very, very good health but sadly the effects of time on their eggs is remorseless,'' he says.

''Very occasionally you see a successful, high-profile woman in the media who has conceived naturally. What you don't see is the successful, high-profile women who failed to conceive.

''Many women who are seen to have conceived later in life may have done so with donated eggs.''

Associate Professor Illingworth says most of his female patients in their 40s are there because they formed a relationship later in life, and not because they deliberately delayed pregnancy.

''I completely agree with Collette Dinnigan,'' he says. ''We recommend that women should try to start a family as soon as they are in a position to do so.

''If they are under 40 and they don't conceive within 12 months, they should have some tests. If they are over 40, they should have tests after six months.''

He warned that women who smoke reach menopause three to four years earlier than non-smokers and advised that women can ask their GP to order a blood test (AMH test) that will estimate how many eggs they may have left.