Autumn is the time of year most associated with large numbers of new babies being born, and according to an Israeli study there may be a scientific reason for it: Human sperm are generally at their healthiest in winter and early spring.
Based on samples from more than 6,000 men treated for infertility, researchers writing in the American Journal of Obstetrics & Gynecology found sperm in greater numbers, with faster swimming speeds and fewer abnormalities in semen made during the winter, with a steady decline in quality from spring onward.
“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,” wrote lead researcher Eliahu Levitas, from Israel’s Ben-Gurion University of the Negev.
If there is a seasonal pattern, that knowledge may “be of paramount importance, especially in couples with male-related infertility struggling with unsuccessful and prolonged fertility treatments,” researchers said.
For the new study, Levitas and his colleagues collected and analyzed 6,455 semen samples from men at their fertility clinic between Jan. 2006 and July 2009. Of those, 4,960 were found to have normal sperm production and 1,495 had abnormal production.
The WHO defines anything above 16 million sperm per milliliter of semen as normal.
Taking into account the approximately 70 days it takes for the body to produce a sperm cell, the study found that men with normal sperm production had the healthiest sperm in the winter.
The men with normal sperm counts produced about 70 million sperm per milliliter of semen in the winter. About 5 percent of those sperm had “fast” motility, or swimming speed, which improves chances of getting pregnant. That compared with the about 68 million sperm per milliliter the men produced in the spring, of which only about 3 percent were “fast.”
However, for men with abnormal sperm production, the pattern did not hold. Those men showed a slight trend toward better motility during the fall and made the largest percentage of normal sperm — about 7 percent — in the spring.
“Based on our results the [normal] semen will perform better in winter, whereas infertility cases related to low sperm counts should be encouraged to choose spring and fall,” the researchers wrote.
Previous studies, mostly in animals, found similar results in line with those species’ breeding seasons, said Edmund Sabanegh, a urologist not involved in the study.
“The hard part of this is really sorting out what factor is accounting for this,” Sabanegh said.
In animal studies, seasonal changes in sperm production and fertility have been linked to factors ranging from temperature to length of daylight exposure.
Among people, previous research has found that sperm counts are falling around the world. While no one knows why, theories range from a more sedentary lifestyle to chemicals in the environment.
Sabanegh said he does not think doctors will start telling men with low sperm counts to wait until winter or spring to try to conceive.
“We would continue to encourage them to try regardless of the season and they may benefit from interventions or treatments,” he said.