One of the reasons for the current preoccupation with Nicole Kidman's growing baby bump, so obvious at the Oscars, could be that it's tangible proof of the fastest-growing trend in the western world: being pregnant in one's forties.

More middle-aged women than ever before are swapping motherhood for the menopause, which can occur any time between 40 and 55. And while the 41-year-old Ms Kidman is certainly not the first celebrity to look forward to motherhood in her final decade of fertility - Madonna had Rocco just before her 42nd birthday, Jane Seymour had twins at 44 and Scots actress Sharon Small is currently pregnant at 41 - the state of late expectancy is not restricted to the rich and famous. It appears that the hoi polloi is also having it all.

According to the Office for National Statistics this week, a staggering 25,400 over-40s in England and Wales became pregnant in 2006, 6.4% more than in 2005 and 109% more than 1991.

In Scotland, figures from the General Register Office for 2006 show the number to be slightly lower at 1833, but this is nonetheless significant because it represents a three-fold increase from the 593 deliveries to fortysomething mothers 30 years previously.

This is less likely to be down to enhanced natural fertility than to increased access to IVF or other assisted-conception techniques. More women are building their careers in their twenties and thirties before embarking on motherhood. Denise Hawkes of the Institute of Economics and author of the research, said: "Women who give themselves more years of experience in the workplace before having children tend to come back to work quicker after their maternity leave. Because they are more connected to their employer, it's easier for them to come back part-time if they want to, rather than leave the labour market."

This is supported by the Scottish Government's Growing Up in Scotland study which found that women in higher income brackets are more likely to have planned pregnancies than those on a lower income. This new development is, naturally, playing havoc with the traditional demographic. For example, there's less likelihood of the older mother's child getting to know its grandparents, and more chance it will be born already an aunt or uncle.

But according to obstetricians, late motherhood - however achieved - can have an adverse effect on health. Older mothers suffer more problems during pregnancy - higher rates of stillbirth, ectopic pregnancy, haemorrhaging and pre-eclampsia - and are thus more likely to require Caesarean sections. Worse, their age means they may be battling with arthritis, depression, cancer and heart attacks as they bring up their children.

All of this, says Dr Victoria Brace of Aberdeen Maternity Hospital, puts more pressure on maternity units, and means there should be more government funding and training for enough consultants.

Eileen Mitchell, an Aberdeen teacher who became pregnant naturally with her second son at 42, remarks: "When I was having my second son the average age of the other women on the ward was 36, and two were 38 - the age at which I had my first son. Even though there were risks associated with high blood pressure, I opted for zero intervention because I just wanted to experience his birth. I never thought I would become a mother and I spend every day counting my blessings. I would not deter anybody from going for it if they are lucky enough."