Men produce the higher quality sperm in autumn and spring, according to a large study in the USA.
After analysing sperm samples from 29,000 men given over 17 years, the researchers found that the strongest sperm was produced in spring. The peak for the greatest number of normally shaped sperm was in the autumn.
The study's lead author Dr Taraneh Nazem, from Icahn School of Medicine in Mount Sinai, New York said: 'It is possible that male fertility is better in these seasons but further research is needed to know how these findings affect pregnancy success rates.'
The researchers believe that sperm quality could be lower at Christmas and during the summer holiday season as men are more likely to drink more and exercise less. They also suggest that the milder temperatures of spring and autumn could also help sperm stay cooler and avoid damage.
Dr Nazem suggested that men should try to maintain consistent diet and exercise habits throughout the year.
The research, which was presented at the American Society for Reproductive Medicine conference in Denver, Colorado, is the largest retrospective study of sperm quality to date.
In the spring men had an average of 117 million moving sperm per millilitre, a significant increase from 112 million in the summer. The analysis also showed that the number of normally-shaped sperm was highest in the autumn.
Professor Allan Pacey, an andrology researcher at the University of Sheffield, said: 'This is certainly the biggest study I am aware of, and confirms that sperm quality is at its worst in the summer months. It might make a small difference to the chances of pregnancy, although more research is needed.'