Some aspiring mothers may think they have heard it all before about Down's Syndrome — warnings about focusing 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% to 15% 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 and birth defects. 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 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% 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%, she said.

“The proportion of women who had a termination has not changed since 1989 at 92% 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 was “very challenging”.

Some experts have warned of an impending fertility crisis as women delay childbearing. 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% in the over-40s and 4% in the over 44s — that is an 89 and 96% failure rate respectively. 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. 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 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. People aren’t paying this enough attention.”