Some aspiring mothers may think they have heard it all before – warnings about focussing on careers and waiting too long to have children. In yet another stark caution this week, doctors said leaving it until 40 defies nature and risks heartbreak. But what makes this latest message more worrying is that it comes amid a row over the increase in Down's syndrome births, which, following a sharp fall after the widespread introduction of antenatal screening in 1989, have since risen.

The Down's Syndrome Association attributed the rise to a greater readiness among parents to accept a baby with the condition and a stronger belief that they had a positive future. That interpretation was based on a survey of 1,000 members of the association and was widely welcomed as an indicator of a more caring society.

Yet it has been challenged by the head of the Down's Syndrome Birth Register, who said a correct reading of the figures pointed to a different conclusion. The number of aborted foetuses with Down's had remained the same, Professor Joan Morris said, and the increase in Down's births was actually due to the increasing age at which women are having babies. Hence, it reinforced the message about the risks of delaying motherhood.

"There has been an enormous shift [in the age of mothers]," Professor Morris said. "It really has been dramatic. Down's is the commonest chromosomal abnormality in babies and the commonest abnormality of any kind in babies born to mothers over 45."

The proportion of mothers aged over 35 has doubled since 1989 from 6 per cent to 15 per cent in 2006, and the group of those over 40 is rising even faster. But the risk of a Down's syndrome pregnancy is 16 times greater in a mother over 40 than in one aged 25. All the risks associated with pregnancy and birth rise sharply over the age of 35, including miscarriage, birth defects and problems such as high blood pressure and diabetes affecting the mother, Professor Morris said. "The risks are fine up to the age of 35 but over 35 they become exponentially greater. That means if you want two or three children, you need to begin by 30. There are consequences of delaying – people should be aware they are trading things off.

"If the risk is one in 100, people will always believe they are going to be among the 99. I have great sympathy with that so long as they are aware. If you are 36 and you put off getting pregnant until you are 37, the chances of you getting pregnant, or of having a miscarriage or a baby with an abnormality, are all higher."

Professor Morris, of the Wolfson Institute of Preventive Medicine, in London, admitted she had been "mildly irritated" by the claim that parents were becoming more accepting of Down's syndrome when it was untrue that more were choosing to keep their babies after discovering the diagnosis. There should have been a 50 per cent rise in Down's Syndrome births from 1989 to 2006 because of the ageing of mothers but the spread of antenatal screening had held the rise at only 4 per cent, she said.

"The proportion of women who had a termination has not changed since 1989 at 92 per cent of those who had an antenatal diagnosis," she said.

Women may have been lulled into a false sense of security by reports of celebrities who have had babies over 40. Madonna had her daughter Lourdes at 38 and her son Rocco at 41. On the other hand, Sarah Palin had her fourth child, Trig, last April at the age of 44. He has Down's syndrome and she admitted discovering the diagnosis had been "very challenging".

Some experts have warned of an impending fertility crisis as women delay childbearing, as the chances of getting pregnant decline with age. One in seven couples has problems conceiving and obstetricians have suggested the "bio-panic" women used to suffer on their 30th birthday has moved to their 40th. They have advised women waiting for "Mr Right" that they maybe ought to settle for "Mr Good Enough" if they want a family. Fertility specialists say too many women are relying on IVF as the answer if things go wrong. Latest figures from the Human Fertilisation and Embryology Authority for 2006 show the live birth rate per cycle is 11 per cent in the over-40s and 4 per cent in the over 44s – that is an 89 and 96 per cent failure rate. In the under-35s it is better – just over two thirds of attempts fail and one third succeed.

Susan Seenan, spokeswoman for the Infertility Network, said: "Women are putting off motherhood and they are entitled to make that choice. But they have to be careful not to depend on IVF. The success rate drops drastically over 40. We would say that if they choose to delay motherhood for whatever reason, [they should] remember IVF is not a guarantee of success, particularly if they are older."

Those making an argument for delaying parenthood point to the economic, social and psychological advantages, including greater maturity and financial security. Children of older parents tend to do better at school, but researchers say this could be because older parents tend to be better off and better educated. On the other hand, raising children is demanding and ageing parents may be less able to cope with the physical and emotional burden. There is also an increased risk that they may die before their children grow up.

Professor Morris said she understood the social pressures on women, having not had her own child until she was 35. "I was lucky. But I know others who have not been. If problems mean you are 37 by the time your baby is born you are facing much higher risks.

"Becoming a mother at 37 is not too bad but if you want a second child you are looking at 39 and that is getting late. I think people are not paying this sufficient attention."

Carla Bradshaw, 42: 'Having kids late is great'

Carla Bradshaw, 42, lives on the Isle of Wight. Married to Paul, she has three children: Kate, four, George, three, and Harry, one.

"I met Paul when I was 27. We talked about children but we enjoyed our careers too much and kept putting it off. When I was 36, I realised if we delayed any longer there could be serious health implications. I spent six months trying to conceive Kate, so it was a relief when I got pregnant at 37. My first boy, George, arrived in just 15 minutes, so I had to deliver him myself at home.

"With all three pregnancies, I had scans and blood tests to check for spina bifida and Down's syndrome but decided against amnio tests.

"For me, having children late has been wonderful. As an older parent I know more about what I want to get out of life, which makes me a stricter and more thoughtful mother."

Paula Nono, 45: 'Down's is no reason for a termination'

Paula Nono, 45, lives in Ealing, west London. She has one son, Alex Priestman, who is 15 months old. He has Down's syndrome. Her husband, James, works for Slough Council.

"I had always been extremely busy with my career, working as a partner in a translation firm. I met my husband at the relatively late age of 39. My attitude was always that it would be nice to have a child, and if it happens, it happens.

"Eventually, we started trying for a baby. It took two years for me to conceive, and I was 43 by the time I finally got pregnant with Alex. We decided that we didn't want to go down the IVF route so I had a couple of months of acupuncture and that seemed to do the trick.

"I knew there was a latent threat in the make-up of my chromosomes: I was told that that any child I had could have Down's, and that the risk was exacerbated by my age. At 16 weeks, I had an amniotic test and it confirmed that our baby would have Down's syndrome. I think I had about half a day of doubt – but no more than that. James had slightly more, but again, not much more than a day really. My protective, maternal instinct kicked in after that.

"I spoke to the Down's Syndrome Association before Alex was born and they helped to convince me that there is a lot of support out there for people in our situation. I now think not only that children born with Down's are wonderful but that Down's is absolutely no justification for terminating a pregnancy.

"I have a wonderful life at home with my boy and an active social life too. Yes, there is a lot of additional pressure and effort involved, with communication classes and special needs playgroups. But James and I have met so many wonderful and inspirational people. Becoming a mother later in life certainly has worked out for me."