Human fertility is in crisis. Our lifestyles have unwittingly rounded on our ability to reproduce, and even with inevitable advances in medicine, experts foresee a dramatic slump in fertility.

The crisis has many fronts. Sperm counts are in freefall, while sexually transmitted diseases and obesity both of which seriously harm our ability to reproduce are rising sharply.

Our environment, stress and the vices we embrace all chip away at our natural fertility. And the trend towards starting families later in life continues, landing more couples in fertility clinics than ever before.

But all is not lost. Few aspects of the human body have been studied as intimately as reproduction, and while much is set by the genetic hand we are dealt at birth, research suggests that changes to our lifestyles can dramatically improve our chances of prolonging our fertility.

Here we bring together the latest scientific research on fertility, and how our life choices affect it.

The impact of smoking on fertility is staggering.

Stop smoking

Last year, a British Medical Association (BMA) report, "Smoking and reproductive life," listed warning upon the detrimental effects of smoking for couples trying to have children. It reduces the chances of conceiving by 10 to 40 per cent per cycle, and is responsible for some 5,000 miscarriages every year.

Men who smoke have lower sperm counts and more malformed sperm than non-smokers. And genetic defects in the sperm can be carried over into children.

A landmark study published last year added weight to many of the warnings raised in the BMA report.

Dutch scientists who followed 8,457 women aged 20 to 40 found that smoking speeds up a woman's reproductive clock, in some cases by 10 years.

"If a smoker is 10 years older, from a fertility perspective, it means a 25-year-old woman has already hit that 35-year-old point at which fertility begins to spiral downwards," said Allan Pacey, a fertility specialist at Sheffield University.

The good news is there is evidence to suggest that smoking-induced damage to fertility may be reversible. While doctors will advise that you can never quit too soon, you can certainly stop too late.

"To improve the situation, women have to be nicotine-free before the egg starts developing and maturing," said Dr Laurence Shaw, deputy director of the Bridge Clinic in London. "If the egg matures while you're still smoking, the chances of successful implantation are much lower. Giving up when your pregnancy test is positive will do nothing to improve your risk of miscarriage," he adds.

Unlike many smoking-related diseases, the effect on fertility is down to nicotine, so nicotine patches can cause all the same problems.

For men, smoking-related sperm damage accumulates over the 90 days it takes for them to form fully, so quitting for three months will ensure no sperm are at risk.

Sexually transmitted infections (STIs) often have few, if any, symptoms, so the damage they cause goes unnoticed until a couple try for children.

Avoid STIs

Chlamydia in particular is on the rise. In 10 per cent of cases, it causes enough damage to the fallopian tubes to render a woman infertile, yet has no visible symptoms.

"Women aged 16 to 24 are most prone to chlamydia, but they tend not to have children at that age, so the first they know about it can be years later," said Professor Bill Ledger, head of reproductive and developmental medicine at Sheffield University.

Many physicians think that even rapid treatment with drugs might not help prevent damage from chlamydia.

Gonorrhea can cause infertility in men and women it blocks the fallopian tubes and, in men, the vas deferens, which carries sperm.

"Disease is a big factor in infertility, and the message is simple," said Ledger. "Use a condom."