Human semen quality may rise and fall in seasonal variation, with the best quality being produced in the winter and spring.
Researchers took semen samples from 6,477 men who had been referred with their partner to a fertility clinic in Israel. Men who had sperm counts in the normal range averaged 71 million sperm per millilitre in spring, which fell to a low of 64 million in autumn. Although total sperm motility was greatest during the summer months, the percentage of fast-moving sperm, thought to increase the chances of fertilisation, was highest in winter.
In men with lower than normal sperm concentration seasonal variation was less marked. The highest percentage of sperm with normal morphology was in the spring, and a slight trend towards greater motility was also observed during this time. However neither the concentration of sperm or the percentage of fast-moving sperm altered significantly between seasons.
The findings have not been directly linked with increased conception rates in the winter although Israeli birth statistics do show an increase in births during the autumn. The authors of the paper suggest that 'the winter and spring semen patterns are compatible with increased fecundability and may be a plausible explanation of the peak number of deliveries during the fall'.
The majority of previous studies on seasonal variations in sperm production have been performed in animals. In pigs, sperm cell development seems to be affected by temperature and in hamsters some of the genes controlling seasonal production have been identified.
Although this study is one of the first to show seasonal sperm quality variation in humans, the authors are confident enough to suggest that men with low sperm concentration should seek fertility treatment in spring and autumn.
However, speaking to Reuters, urologist Dr Edmund Sabanegh, director of the Centre for Male Fertility in Cleveland, USA, who was not involved in the study, disagreed. 'We would continue to encourage [patients] to try regardless of the season, and they may benefit from interventions or treatments', he said.
The research was published in the American Journal of Obstetrics and Gynecology.