Reproductive health experts are urging countries with declining populations to allocate resources and research ways to combat infertility before time runs out.

The call for action is a result of research findings published in the British Medical Journal predicting child-bearing problems to become more prevalent in the coming years due to medical, environmental and social factors.

To date, about 15 percent, or one in seven couples in developed countries already find it hard to conceive a baby. If the trend continues, incidence of infertility could rise to as high as one in three.

"It is right that we should focus resources and research on this issue now because of the likely impact it will have in the future," said Prof Siladitya Bhattacharya, head of the department of obstetrics and gynecology at Aberdeen University, Scotland.

He added, "As well as the medical factors, the social effects of women deferring childbirth will also affect fertility rates. Most expect to stop using contraception and become pregnant immediately, but it rarely works like that."

Researchers from the University Hospital in Denmark and the University of California, said the best way to counteract infertility was to deal with avoidable causes.

One study found that men with the sexually transmitted disease chlamydia had a three-fold higher level of DNA damage in their sperm than men without the infection.

It is also known that obesity is linked to fertility problems.

Incidences of Chlamydia have doubled over the last decade - and 6% of girls under the age of 19 are currently classed as obese.

"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," explained Prof. Bill Ledger of Sheffield University.

The British fertility expert adds that inflexible work hours and financial and career aspirations cause many women to put off having children until they are in their late 30s and early 40s.

"Women are simply not as fertile after 35. 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," Prof. Ledger had said.