By Michelle Roberts
BBC News health reporter in Copenhagen
Infertility is set to double in Europe over the next decade, a leading UK fertility expert has warned.
Infertility rates 'could threaten Europe's population'
One in seven couples now has trouble conceiving naturally, but Professor Bill Ledger from Sheffield University warned this could rise to one in three.
He told a European fertility conference that women should be offered career breaks so they could have children younger, when they are more fertile.
Obesity and sex infections were also increasing infertility, he said.
The incidence of chlamydia, a sexually transmitted infection which carries a risk of infertility, has doubled over the last decade - and 6% of girls under the age of 19 are currently classed as obese.
A potential rise in male infertility could also affect couples, Professor Ledger said. Both the quality and quantity of sperm appeared to be in decline.
"Young people of today will become tomorrow's patients in infertility clinics," Professor Ledger said.
He warned the rise in sexually transmitted infections in young teenagers was likely to cause blocked fallopian tubes in some.
"Later, when these young women want to become mothers, they find they can't conceive."
Professor Ledger added: "The obese child is almost certain destined to become an obese adult. Many women who are overweight will not ovulate as efficiently."
'Too few children'
Inflexible working hours and financial and career aspirations mean many women are putting off having a family until they are in their late 30s and early 40s, he said.
"The sustainability of the population of Europe is at risk because there are too few children being born. It is a threat to the future."
But he said it was not too late to reverse the trend, with many countries, such as those in Scandinavia, introducing policies to encourage women to have children earlier.
He suggested the UK also follow the lead of France by introducing tax relief and giving greater support to women who want to take career breaks to start a family.
"Women are simply not as fertile after 35," Professor Ledger said.
"It's easier and more straightforward to do whatever you can to encourage women to have children naturally, rather than waiting to the point at which IVF may be needed."
Dr Allan Pacey, of the British Fertility Society, said: "Nature designed women to have children in probably their late teens and early twenties, and many women are now waiting until they are over 35.
"The message has to be driven home that the sooner you do it, the more likely it is you will be able to conceive without medical assistance."
Dr Pacey said the NHS was unlikely to be able to fund a huge increase in demand for fertility treatment. He also stressed that treatment was not without risk.
Dr Becky Lang, from the Association for the Study of Obesity said the issue of fertility and obesity was often overlooked.
"Being obese can significantly reduce your fertility as well as causing more complications when they do become pregnant.
"We have just been asked by the NHS to conduct more research into this as it is of growing concern to health professionals."
A spokesperson for the Department of Health said: "The government is committed to improving the health of nation, reducing obesity, promoting healthy living, increasing physical activity and tackling sexually transmitted infections."