The findings could matter for couples trying to conceive and for fertility clinics interpreting test results.
Men’s sperm move more quickly in summer than in winter, according to a large new study analysing more than 15,000 semen samples.
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Researchers in Denmark, the United Kingdom, and Canada analysed semen samples from donors in Denmark and the United States.
They found clear seasonal changes in semen quality, measured by the concentration and number of progressively motile sperm, the sperm that swim forward effectively and are most likely to fertilise an egg.
The concentration of the fastest-moving sperm rose through spring, peaked in early summer and then fell to its lowest levels in mid-winter.
The findings could matter for couples trying to conceive and for fertility clinics interpreting test results. A semen analysis taken in summer may look different from one taken in winter.
However, the differences do not mean men are fertile only in summer or infertile in winter.
Overall, sperm production did not change across the year.
The total number of sperm and the volume of semen remained stable in all seasons. What varied was how well the sperm moved.
The total number of sperm and the volume of semen did not change with the seasons. That suggests that sperm production itself remains stable throughout the year.
Instead, what appears to vary is how well sperm move.
Scientists have long known that sperm development is sensitive to temperature.
However, previous studies have often suggested that sperm perform best in colder months, but results have been mixed and many involved smaller groups of men.
By analysing a large number of samples from two countries with very different climates, the new research suggests the seasonal pattern may be more consistent than previously thought.
The study was observational, meaning it shows a pattern but cannot explain exactly why it happens.
