Seasonal Trends in Sperm Quality in Denmark and Florida

Background Spermatogenesis is a temperature-sensitive process, but previous studies on seasonal variation in semen quality have produced conflicting results, often due to differences in sample size, methodology, or local climate. Most studies have reported peak sperm motility in winter months, although findings vary by country. This study investigated whether the quality of semen varies seasonally in Denmark and Florida, and whether ambient temperature contributes to any seasonal trends. Design This retrospective observational study included data from 15,125 candidate sperm donors collected between 2018 and 2024. The cohort comprised 10,670 men from four Danish cities (Aarhus, Aalborg, Odense, and Copenhagen) and 4,911 men from Orlando, Florida. Participants were aged 18–45 years and resided near the collection sites. All ejaculates were analysed within one hour of collection using the same computer-assisted semen analysis (CASA) system and standardised temperature-controlled protocols. Semen parameters assessed included ejaculate volume, sperm concentration, concentrations of progressively motile sperm (‘grade a’ and ‘grade b’), and total motile sperm count (TMSC). The effects of monthly mean outdoor temperature during the month of collection and two months earlier (representing early spermatogenesis) on sperm parameters were also modelled. We used nonlinear statistical methods (GAMs) to accurately display the month-to-month seasonal variation, while accounting for the nonlinear effects of male age, monthly mean temperature, and year of study on semen parameters. Results Strong and consistent seasonal variation was observed in the concentration of progressively motile (‘grade a’ and ‘grade b’) sperm in both Denmark and Florida. The concentration of ‘grade a’ sperm was highest from May to July and lowest between October and March in both countries. Despite climatic and demographic differences, seasonal trends were remarkably similar. No evidence of seasonal variation was found in ejaculate volume or total sperm concentration, indicating that the rate of spermatogenesis did not vary seasonally. TMSC also varied seasonally, even after controlling for average daily temperature in the contemporaneous and preceding months, suggesting that other seasonal factors, possibly related to lifestyle or other environmental factors, may affect sperm motility. Conclusion Semen quality, particularly sperm motility, exhibits clear and consistent seasonal variation in both temperate and subtropical climates. These findings highlight the importance of accounting for seasonality when assessing semen quality for fertility evaluation and indicate that seasonal variation persists even in warm climates such as Florida. Trial registration number: Not applicable.