Men hoping to settle down and start a family should give up smoking, a new study warns. Adding to the long list of dangers from tobacco, researchers found that heavy smokers end up about two-thirds less fertile than non-smokers.

Dr. Lani Burkman, a fertility expert at the University of Buffalo School of Medicine who led the study, said that smoking does not make men sterile, but husbands who chain smoke could add a couple of years of futile attempts before their wife is able to get pregnant.

"Their fertility potential is drastically reduced," said Burkman.

The study, presented at a recent meeting of the American Society of Reproductive Medicine, compared sperm samples from 18 smokers to a similar group of men who never smoked. As part of the test, researchers divided the protective layer that covers female eggs in half and placed them in a separate lab dish.

The smoker's sperm samples were tested against one half of the egg covering, while the non-smokers were tested against the other half. Success was judged by the ability of the sperm to stick to this outer coating.

In the time it takes reproductive cells to work their magic, two-thirds of the sperm samples choked by tobacco smoke failed to pierce the outer layer of the female egg, suggesting that smokers would have a tougher time causing a pregnancy. Overall, men who smoked 18 or more cigarettes a day for at least two years had about one quarter of the fertilization power as non-smokers.

Those who smoked less had better functioning sperm, suggesting that guys may not have to go cold turkey to improve their chances.

"If we can get men to cut down to five or six cigarettes a day, they can increase their fertility," said Burkman.

Just the same, the findings point to yet another reason to kick the habit, the sooner the better.