(Reuters) - Autumn is the time of year most associated with bumper crops of new babies, 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 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 Ben-Gurion University of the Negev in Beer-Sheva.

If there is a seasonal pattern, they said, that knowledge may "be of paramount importance, especially in couples with male-related infertility struggling with unsuccessful and prolonged fertility treatments."

For the new study, Levitas and his colleagues collected and analyzed 6.455 semen samples from men at their fertility clinic between January 2006 and July 2009. Of those, 4,960 were found to have normal sperm production, and 1,495 had abnormal production, such as low sperm counts.

The World Health Organization defines anything over 16 million sperm per milliliter of semen as a normal sperm count.

Taking into account the approximately 70 days it takes for the body to produce a sperm cell, the researchers found that men with normal sperm production had the healthiest sperm in the winter.

For example, those men produced about 70 million sperm per milliliter of semen during the winter. About 5 percent of those sperm had "fast" motility, or swimming speed, which improves a couple's chance of getting pregnant.

That compared to the approximately 68 million sperm per milliliter the men produced in the spring, of which only about 3 percent were "fast."

For men with abnormal sperm production, however, the pattern didn't hold. Those men showed a slight trend toward better motility during the fall and made the largest percentage of normal shaped sperm - about 7 percent - during 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, have found similar results in line with those species' breeding seasons, said Edmund Sabanegh, a urologist who was not involved with the new research.

"The hard part of this is really sorting out what factor is accounting for this," said Sabanegh, the chairman of the urology department at Ohio's Cleveland Clinic.

In animal studies, seasonal changes in sperm production and fertility have been linked to factors ranging from temperature, to length of daylight exposure and hormone variations.

Among people, previous research has found that sperm counts around the world are falling. While no one knows why, theories range from a more sedentary lifestyle to chemicals in the environment that affect sperm health.

Sabanegh said he doesn't think doctors will start telling men with low sperm counts to wait until the winter or spring to try to conceive a child.

"We would continue to encourage them to try regardless of the season, and they may benefit from interventions or treatments."