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Barreca, a member of the UCLA Institute of the Environment and Sustainability, observed that there tend to be fewer U.S. births in April and May, nine months after the warmest part of the year.
“If you look nine months after a heat wave in August, the following May you see significantly fewer births,” Barreca said.
It’s not that people have less sex in hot weather. (In fact, unpublished research by Barreca indicates people actually have more sex when temperatures rise.) Rather, the pattern is likely due to heat’s effect on male fertility: Studies show that sperm production falls in hot weather, he said.
The study, which was published in the journal Demography, found that high temperatures have a significant negative effect on fertility and birth rates and that cold temperatures have relatively little impact. The research projects that as climate change drives temperatures up and increases the number and severity of heat waves, getting pregnant may become harder than ever.
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Excessive heat affects sperm production
While the rest of the body may feel the chill that comes with the end of the year, sperm are quite happy without the excessive heat. Men are advised to avoid heat around the testicles when trying to conceive. A lower temperature is needed for optimum sperm production. If it’s too warm, it can slow production and cause a higher number of abnormal sperm.
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The ideal environment for sperm is four degrees cooler than the body. A number of things can cause that temperature to go up, like:
- Wearing briefs instead of loose boxers
- Wearing tight-fitting clothing
- Sitting with a warm laptop on your lap
- Relaxing in a hot tub or sauna
- Fever and other health issues
Higher testosterone means higher fertility
Testosterone levels are at their highest during colder months. That leads to a rising libido. A stronger sex drive combined with more testosterone means higher fertility and a better chance of conceiving. Studies found that sperm concentration is lowest in August through October and then picks up again as the temperature cools off.
November and December were identified as the most fertile months, with the highest number of births nine months later.