Weather 
Maybe every summer is the summer of 69.
New research reports that summer is the best season for swimming — and not just in the pool. Turns out that sperm motility may actually be higher at certain times of the year than others, a finding that could lead to changes in guidance for couples trying to conceive — and even in the way sperm banks are run.
Scientists affiliated with three universities across the UK, Canada and Denmark analyzed semen samples collected between 2018 and 2024 from over 15,000 men who had applied to be sperm donors at Cryos International clinics in Denmark and Florida.
They assessed three criteria across all months of the year: ejaculate volume, sperm concentration and sperm motility, or how well the sperm can swim and propel themselves forward.
While seasonal changes did not appear to impact total sperm concentration and ejaculate volume, the effects were noticeable on the sperm’s speed.
In dramatically different locales — northern Europe and the southern US — the fastest sperm were in their highest numbers during June and July and at their lowest in December and January.
Clearly, it’s not just summer temperatures that account for this discrepancy, co-author Allan Pacey said.
“We were struck by how similar the seasonal pattern was in two completely different climates,” he said. “Even in Florida, where temperatures stay warm, sperm motility still peaked in summer and dipped in winter, which tells us that ambient temperature alone is unlikely to explain these changes.”
The findings could point to certain lifestyle factors, like exposure to daylight or other environmental shifts, playing a larger role in sperm success than previously thought.
Notably, the study results don’t line up with conventional thinking about fertility and childbirth.
Countless motherhood and baby blogs claim that fall and winter are the best times to conceive.
And some data reflects this theory as well. The CDC notes that in 2023, the most babies were born in August, while the fewest were born in February. This would mean the most babies for that year were conceived in the late fall and early winter.
Additionally, a 2020 study of menstrual cycles found that fall and winter had the highest fecundability, or probability that babies would be conceived.
The authors of this most recent research even acknowledged that most other studies “have reported peak sperm motility in colder months” and cautioned that “differences in sample sizes, methodologies or local climates” may have played a role in the conflicting results.
Regardless, these new findings contribute valuable insights into an ancient biological process that still mystifies medical professionals to this day.
Pacey said that moving forward, it will be important to consider seasonality when evaluating semen quality, suggesting that fertility clinics may start looking at seasonal patterns to best treat patients who are trying to conceive.
Ultimately, he said, “these findings deepen our understanding of male reproductive health and may help improve fertility outcomes.”
