The size of a man's stomach matters to the quality of his sperm, researchers say.

Fertility researchers found the more men weigh, the more likely they are to have poor quality sperm.

The study looked at more than 1,500 Danish men with an average age of 19. The men volunteered to give a semen sample during their mandatory exams for military service.

Overweight men are more likely to have poorer quality sperm.

Severely obese men were less able to successfully impregnate women, Dr. David Ryley of Boston's Beth Israel Deaconess Medical Center told a fertility conference in Philadelphia.

Ryley and his colleagues accounted for differences in weight by height.

They took measurements such as sperm concentration, shape and motility, as well as testicle size and hormone levels.

Sperm counts and sperm concentration were about 22 per cent and 24 per cent lower respectively in overweight men compared to men with a healthy weight, the researchers found.

Estrogen balance

Since fat cells produce estrogen, excess fat may harm sperm, scientists said.

Being too thin was also a problem. Among underweight men, sperm counts and concentration were about 28 per cent and 36 per cent lower than in healthy weight men.

The male results appear in the September supplement of the journal Fertility and Sterility.

A second study by Ryley came to a similar conclusion for women.

Researchers looked at women who had 5,847 attempts at in vitro fertilization, in which embryos are fertilized in a lab dish.

Obese women had little more than a one in five chance of becoming pregnant by in vitro fertilization, compared to one in four among women with healthy weight.

Unlike previous studies, the team did not find very thin women had any more trouble conceiving than others.

The female study appears in the October issue of the journal Fertility and Sterility.