When a couple has trouble conceiving a baby, statistics show that the problem is only slightly more likely to be on the woman's side than on the man's.
There are a number of factors that can impair male fertility, but one of the most common is blockage of the tubes that carry the sperm.
Like in women, this blockage can be scar tissue caused by chlamydia, a sexually transmitted disease that often has no symptoms until the damage to fertility has been done.
Another common problem is called "varicocele.
" In one study it was found in 14% of the men seeking help for infertility.
A varicocele is a twisted, enlarged varicose vein in the connective cord of the testicles.
These are found in as many as one-fifth of all men, and do not always impair fertility.
In fact, it is unknown just what impact they have, but they are more common in men seeking help for fertility than in the general male population.
Treatment for a varicocele is surgery, which yields about a third of the couples getting pregnant within the first year after the surgery.
The common childhood disease, mumps, can cause lowered fertility in men if it is contracted after puberty.
Other infections, such as glandular infections, can damage sperm or otherwise impair normal fertility.
As with women, obesity is thought to affect male fertility adversely.
Drug side effects and chronic medical conditions are two other causes of male infertility.