These findings could be essential not only for couples dreaming of starting a family but also for reproductive medicine clinics and their test interpretations.
An international team of researchers from Denmark, the UK, and Canada analyzed over 15,000 sperm samples from donors in Denmark and the US, according to EuroNews. The results revealed surprising seasonal changes.
The scientists focused on the concentration of so-called progressively motile sperm, which are the ones that can swim forward effectively and have the best chance of successfully fertilizing an egg. Data showed that the concentration of these "fastest swimmers" starts to rise in spring, peaks at the beginning of summer, and then drops to its lowest in the middle of winter.
These differences certainly don't mean that men are only fertile in the summer and infertile in winter. Interestingly, sperm production itself doesn’t change throughout the year. The total number of sperm and the volume of ejaculate remain stable in all seasons. What actually changes is solely the quality and efficiency of their movement.
This finding adds important context for practice, as sperm test results taken in the summer can look different from those taken in the winter. Although science has long known that sperm development is temperature-sensitive, previous smaller studies provided mixed conclusions, often even suggesting that sperm do better in cooler months.
