Women who delay trying to have a family cannot rely on IVF to compensate for declining fertility, scientists warn.
The researchers evaluated women's chances of conceiving
The French Institute of Health and Medical Research used computer models in a study of fertility at various ages.
The research found births to over-40s through IVF only slightly more common than they would have been naturally.
In an article in Human Reproduction, under-35s trying for a baby are urged to "be patient" while over-35s should "be impatient" and try IVF sooner.
Professor Henri Leridon, a demographer from the French Institute of Health and Medical Research looked at women's odds of conceiving naturally aged 30, 35 or 40.
He then assessed women's chances of conceiving if they began IVF at 34, 38 and 42 - assuming women underwent two cycles of treatment.
The computer model combined the monthly probabilities of conceiving, the risk of miscarriage and the probability of becoming permanently sterile through age.
It showed that three-quarters of women aged 30 who try to concieve naturally will do so within a year.
This falls to two-thirds of those starting at 35 and 44% of those starting at age 40.
Four years after trying to start to conceive naturally, 91% of 30 year-olds, 84% of 35 year-olds and 64% of 40 year-olds will have been successful.
Professor Leridon then looked at women's chances of becoming pregnant through assisted reproductive techniques - assuming women would wait less time to start IVF the older they were.
He said the results would mean that out of 100 women trying to become pregnant at age 30, 91 will have a child within four years naturally, another three will do so during the next two years thanks to IVF and six will remain childless.
Out of 100 starting at age 35, 82 will have a child after three years, another four thanks to assisted reproduction treatment (ART) and 14 will remain childless.
Of 100 starting at age 40, 57 will have a baby after two years, another seven after ART and 36 will remain childless.
For those over 40 the number of births through ART is only slightly higher that it would be if they had tried for another two years to conceive naturally.
Professor Leridon said the study had been carried out because it was important to answer a range of practical questions about the chances of either natural or artificially aided conception so that couples could be given more precise information about the potential risks of postponing childbirth beyond a given age.
He added: "The take home message if you are under 35 is 'be patient'. Even if you fail to conceive within a year your chances of conceiving subsequently are still substantial.
"More than half of you who are childless after one year will conceive during the next two years.
"But, if you are 35 or over the message is 'be impatient'. Your changes of conceiving naturally are still significant, but in the case of failure, assisted reproduction technologies will not fully compensate you for the years, and the chances of conceiving, that you have lost."
Professor Bill Leger, of the Royal College of Obstetricians and Gynaecologists told BBC News Online: "Women in their 40s are rarely very successul at becoming pregnant naturally.
"But women of around 40 have poorer egg quality, which is a problem for IVF."