Currently men have just two birth control options - condoms or a vasectomy.
It has long been known obesity is associated with reduced semen quality. But studies into the affect of weight loss on sperm have been too small...
Underlying factors like hormone imbalances could be fueling the tumors.
With the increasing cure rate of pediatric cancers, infertility has become an important concern for patients and their families, said the researchers.
"The most heart-wrenching response is tears, when we tell them 'you're pregnant!' and they cry."