Women who wait until their late 30s to have children risk heartbreak, warn doctors.
Specialists led by Susan Bewley, who treats women with high-risk pregnancies at Guy's and St Thomas' Hospital here, warned that age-related fertility problems increase after 35 and dramatically after 40.
Over the last 20 years, pregnancies in women over 35 have risen markedly and the average age of mothers has gone up, reports the online edition of BBC.
Writing in the British Medical Journal, the London-based fertility specialists say the best age for pregnancy remains 20 to 35 and women who defy nature risk heartbreak.
The specialists are "saddened" by the number of women they see who have problems.
They write: "Paradoxically, the availability of IVF (In vitro fertilization) 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.
Clare Brown, chief executive of Infertility Network, Britain, 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."