A team from Aarhus University in Denmark tracked the health of more than 300 women and their sons. Although the majority of the women were of normal weight when they became pregnant, 25 had body mass indexes classed as overweight or obese.
Tests showed their sons tended to have slightly lower concentrations of sperm, as well as fewer mobile sperms, the online edition of Daily Mail reported.
It was possible that higher levels of the hormone oestrogen, associated with being overweight, might harm the development of male foetus reproductive organs, the scientists said.
However, the differences were so small that the researchers could not be sure they were not due to chance. The researchers said further studies should be carried out.
Obesity is also known to pose a major risk for chronic diseases, type-2 diabetes, cardiovascular disease, hypertension and stroke and certain forms of cancer.