Being overweight may reduce a man's chances of fathering a baby, according to a study.
The findings are based on tests on 500 men
Scientists in the United States say they have found a link between a man's weight and the quality of his sperm.
They believe sperm starts to deteriorate as soon as men go beyond a healthy weight for their size.
They have suggested that men who become obese may no longer be able to father children because the quality of their sperm is so poor.
Dr William Roudebush and colleagues at Reproductive Biology Associates, a private IVF clinic in Atlanta, analysed the sperm of 500 men.
They found a direct correlation between their body mass index (BMI) - a measure of their body fat based on their weight and height - and sperm volume and quality.
"As men allow their BMI to increase we certainly did see a correlation with their semen parameters," Dr Roudebush said.
According to the researchers, becoming obese reduces a man's sperm count below the level needed to fertilise an egg.
Even if he does manage to fertilise his partner's eggs there is a far greater chance of miscarriage because the quality of the sperm is reduced, the researchers said.
But they also warned there may be risks for men who are even slightly overweight.
Speaking at the American Society for Reproductive Medicine's annual meeting in San Antonio, Texas, they said their findings may explain why male infertility is on the increase pointing to record rates of obesity in many western countries.
Professor Harry Moore of the University of Sheffield is carrying out a study to examine the impact of chemicals in the environment on male fertility.
He said further research is needed to back up these findings.
"We are still unsure why a lot a men are infertile beyond the fact that they have lower sperm counts and their sperm motility isn't very good," he told BBC News Online.
"A lot of it is chance. It appears to be a combination of genes and the environment."