A formula that predicts a woman’s chances of pregnancy has been devised by scientists.
It combines information about how fertility drops with age with the length of time a woman has been trying to start a family, to come up with their odds of conceiving.
For example, they have worked out that the average 25-year-old who has been trying to get pregnant for six months has a 15 per cent chance of doing so in the following month.
By the age of 30, her odds are 13 per cent and, at 35, they have dropped below 10 per cent.
The speeding up of the biological clock mean the chances of pregnancy plummet after 35.
The average 40-year-old who has been trying for six months has just a 5 per cent chance of getting pregnant in the next month – or odds of one in 20.
The calculations also show that when a woman is 25, it will take 13 months for her odds of conceiving quickly to fall below ten per cent.
But a 35-year-old woman has just six months before her chances are so slim.
The long-standing rule of thumb is that those trying for a family should wait a year before seeking help, although doctors are increasingly acknowledging the impact of age.
The researchers, from the University of Warwick and the London School of Economics, say that more detailed information could make it easier for couples to discuss fertility issues with their GP.
Professor Geraldine Hartshorne said: ‘People feel embarrassed and upset and don’t want to go to the doctor. Men, in particular, can be a little bit reluctant.
‘As time goes by and people have been trying for a while, they start to get stressed and upset and that can affect their chances of having sex and then becoming pregnant. Approaching a doctor about a personal matter is daunting, so knowing the right time to start investigations would be a useful step forward.’
Writing in the journal PLoS One, she also warns that taking too long to conceive could indicate that the resulting pregnancy might be risky.
The work could help doctors to decide whether to refer patients for costly and uncomfortable tests or advise them to keep trying for a baby a little longer.
The researchers have passed their work to the National Institute for Health and Clinical Excellence, which formulates health guidelines. In future, it may be possible to create an online calculator that provides couples with a personalised prediction.
Professor Hartshorne added that factors such as smoking or being fat are ‘not the most important things’ when it comes to conceiving. However, a healthy lifestyle will boost the odds of a healthy baby.
She said: ‘If your tubes are blocked, giving up smoking really isn’t going to make a difference, but things like smoking and obesity do have important effects when you do get pregnant and in that respect they should be addressed as soon as possible.’
Scientists have succeeded in turning laboratory-made eggs into baby mice for the first time.
Experts have described the work as ‘incredible’ and point out that the baby mice born as a result of the experiment went on to have litters of their own.
The research at Kyoto University in Japan could eventually allow women left infertile by cancer treatment or by premature menopause to ‘grow’ new eggs.
Dr Allan Pacey, a Sheffield University fertility expert, said the Japanese work, reported in the journal Science, could one day lead to the ‘routine’ production of new eggs.