Some were given false hope by celebrity mothers having babies in their 40s, with some not admitting they had used donor eggs, they suggested.
The study, presented at the European Society of Human Reproduction's annual conference in Lisbon, shows that at the age of 38 around 24 per cent of women having fertility treatment went on to have a baby.
By the age of 44 the figure was just 1.3 per cent, with a significant decline in live birth rates seen between the ages of 41 and 42.
In all, a woman of 38 trying for a baby was 18 times more likely to succeed than a woman just six years older, the figures show.
The 12-year study involved 4000 women. Dr Marta Devesa, lead researcher, said women should be encouraged to start a family earlier.
"There is a clinically relevant decline from 41/42 - but the prognosis is really futile from 44 and onwards."
Professor Charles Kingsland, from Liverpool Women's Hospital, said many well-educated women knew little about their fertility levels.
"Even the most well-informed women are ignorant and still just don't realise how much age affects fertility."