It appears that human sperm, which take about 10 weeks to develop are acutely sensitive to temperature. I recently read a study in Science News that showed that would-be fathers who expose themselves to excessive heat in the weeks before they conceive children place their future offspring at unnecessary risk of brain cancer. How big was the risk? Fathers of children with brain cancer were 3.4 times as likely as other dads to have used a sauna and 2.1 times as likely to have had exposure to at least one heat source during the 3 months prior to conception.
Mothers do not appear to be affected by the heat exposure – probably because the ovaries are much more protected deep inside the pelvic cavity.