Kenneth Glad
A large international dataset is pointing to subtle seasonal changes in sperm health.
So does timing really matter for fertility, or is the effect too small to notice?
Questions about the study’s importance emerged almost immediately. Dr. Sherman Silber, a U.S. fertility specialist not involved in the research, cast doubt on its practical relevance.
“The differences reported in this study are ‘very, very tiny’ and ‘make no difference whatsoever biologically,’” he told Live Science.
Despite that criticism, the research, published in Reproductive Biology and Endocrinology examined more than 15,000 semen samples from donor applicants in Denmark and Florida, all aged 18 to 45.
The results suggest sperm motility follows a seasonal rhythm, rising around the middle of the year and falling during colder months.
Researchers used computer-assisted analysis to measure semen volume, concentration and how effectively sperm swim forward.
The consistency of the pattern across two distant regions is one of the study’s more notable findings, even if its implications remain debated.
Causes remain uncertain
To understand what might drive the trend, scientists explored environmental factors in the period leading up to sample collection. However, no direct relationship between temperature and sperm quality was identified.
“The fact that the seasonality still existed when we accounted for ambient temperature made us think that other lifestyle changes might be important,” said Allan Pacey, a study co-author and professor of andrology at the University of Manchester.
“This could include diet, exercise, exposure to sunlight. But we did not measure these things so we can only speculate.”
This leaves open the possibility that seasonal habits, rather than climate itself, are influencing the results.
Variations in physical activity, nutrition or time spent outdoors could all play a role, though the study did not directly test these factors.
Broader context and debate
Research from other parts of the world has not produced a single clear pattern. Some European studies report similar mid-year peaks, while findings from China suggest sperm quality may rise at different times of the year.
Such inconsistencies point to the influence of local conditions and behaviours, rather than a universal biological cycle. They also make it harder to draw firm conclusions about how, or if, individuals should respond.
Some scientists have suggested the pattern could be a faint evolutionary leftover, linked to seasonal breeding seen in other species. Humans, however, are far less dependent on such cycles, which may explain why any effect appears modest.
For now, the findings add to a growing body of research without settling the question. Whether seasonal shifts in sperm quality have any meaningful impact on fertility remains unclear.
Sources: Live Science, Reproductive Biology and Endocrinology
