Women who wait until their late 30s to have children are defying nature and risking heartbreak, leading obstetricians have warned.
Pregnancies in women over 35 have risen markedly
Over the last 20 years pregnancies in women over 35 have risen markedly and the average age of mothers has gone up.
Writing in the British Medical Journal, the London-based fertility specialists say they are "saddened" by the number of women they see who have problems.
They say the best age for pregnancy remains 20 to 35.
Over the last 20 years the average age for a woman to have their first baby has risen from 26 to 29.
The specialists, led by Dr Susan Bewley, who treats women with high-risk pregnancies at Guy's and St Thomas' Hospital, warned age-related fertility problems increase after 35 and dramatically after 40.
Other experts said it was right to remind women not to leave it too late.
'Having it all'
In the BMJ, the specialists write: "Paradoxically, the availability of IVF may lull women into infertility while they wait for a suitable partner and concentrate on their careers and achieving security and a comfortable living standard."
But they warn IVF treatment carries no guarantees - with a high failure rate and extra risks of multiple pregnancies where it is successful.
For men, there are also risks in waiting until they are older to father children as semen counts deteriorate with age, they say.
Once an older woman does become pregnant, she runs a greater risk of miscarriage, foetal and chromosomal abnormalities, and pregnancy-related diseases.
They add: "Women want to 'have it all' but biology is unchanged; deferring defies nature and risks heartbreak."
"Their delays may reflect disincentives to earlier pregnancy or maybe an underlying resistance to childbearing as, despite the advantages brought about by feminism and equal opportunities legislation, women still bear full domestic burdens as well as work and financial responsibilities."
Dr Bewley told the BBC News website: "We are saddened because we are dealing with people who can't get pregnant or are having complications.
"Most women playing 'Russian Roulette' get away with it, most people are fine. But I see the casualties.
"The best time to have a baby is up to 35. It always was, and always will be."
She added: "I don't want to blame women, or make them feel anxious or frightened.
"The reasons for these difficulties lie not with women but with a distorted and uninformed view from society, employers, and health planners.
"Doctors and healthcare planners need to grasp this threat to public health and support women to achieve biologically optimal childbirth.
"Where we can, we should be helping women to have children earlier."
Clare Brown, chief executive of Infertility Network UK, said "Delaying having children until you are in your thirties is a choice many people make but they need to be aware of the added problems when trying to conceive, particularly over the age of 35 when a woman's natural fertility declines.
"When this is exacerbated by a further complication such as blocked tubes or low sperm count the chances of a successful pregnancy even using IVF are much less."
Peter Bowen-Simpkins, of the Royal College of Obstetricians and Gynaecologists, said: "The biological clock is one thing we cannot reverse or change.
"The message that needs to go out is 'don't leave it too late'."