Male personal heat exposures and fecundability: A preconception cohort study.

Plain-language summary

This study asked whether a man's heat exposures affect how quickly his partner conceives, which is a harder and more useful question than whether they affect a semen sample. It followed 3,041 couples in the United States and Canada who were trying to conceive between 2013 and 2021, recorded the men's saunas, hot baths, seat heaters and tight underwear at the start, then tracked pregnancies for up to 12 cycles. The measure is the fecundability ratio, the chance of conceiving in a given month compared with men who had no such exposure, where 1.0 means no difference. Hot baths or tubs three or more times a month came in at 0.87 and a fever in the previous three months at 0.94. Both are small reductions, and both confidence intervals crossed 1.0, which means no effect at all remains a possibility. The clearer signal was in combination: men over 30 who reported four or more heat exposures of any kind had a ratio of 0.60. Saunas, laptop use, seat heaters and tight underwear on their own showed little.

Key findings

• Hot bath or tub use three or more times a month was associated with a fecundability ratio of 0.87, a small reduction whose confidence interval still included no effect.
• Fever in the three months before enrolment was associated with a ratio of 0.94.
• Men over 30 who reported four or more heat exposures of any kind had a ratio of 0.60, the largest reduction observed.
• Saunas, laptop use, seat heaters and tight-fitting underwear individually showed little association with conception.