Sperm production is healthiest in winter and early spring for most men, says a new study from Israel, while men with abnormal sperm production might have better luck in the fall.

Researchers from Ben-Gurion University of the Negev looked at 6455 semen samples from men treated for infertility, and found that sperm production was much greater in the winter, and began declining in quality starting in early spring. Not only was there a higher winter sperm count, but it also swam much faster and had fewer abnormalities.

Autumn is typically a high point for births of new babies, and previous studies in animals had indicated a peak in sperm quality at certain points in the year. Until now, it was unclear whether human sperm production also improved seasonally.

The study, led by Dr. Eliahu Levitas, was published in the American Journal of Obstetrics & Gynecology in February.

The research team collected the semen samples at their fertility clinic from 2006 to 2009. Of the samples, 4,960 had normal sperm production, and 1,495 had abnormal production, such as low sperm counts. Anything over 15 million sperm per milliliter of semen constitutes a normal sperm count.

They found that the men with normal sperm production had the healthiest sperm in the winter, with about 70 million sperm per milliliter of semen at that point. Five percent of the winter sperm had fast motility, or swimming speed, which makes it easier for couples to get pregnant.

In the spring, sperm production declined to about 68 million sperm per milliliter, only three percent of which had fast motility.

However, the men with abnormal sperm production did not show the same seasonal peak. They had slightly better motility in the fall, and made the largest percentage of normal sperm in the springtime.

The researchers concluded that normal semen will perform better in winter, while men with low sperm counts might do better in spring and fall.

"The hard part of this is really sorting out what factor is accounting for this," said Dr. Edmund Sabanegh, the chairman of the urology department at Ohio's Cleveland Clinic, who was not involved in the study, to Reuters.

For now, Sabanegh recommends that men with low sperm counts who are trying to conceive a child should keep trying throughout the year, and take whatever interventions might help them.

Changes in seasonal sperm production can be related to factors like temperature, length of daylight exposure, and hormone variation. Some research indicates that worldwide sperm counts are falling, for reasons that might include sedentary lifestyles or environmental contamination.

The researchers ultimately hope that finding a seasonal pattern to sperm production and health could help couples struggling with male infertility choose the best time to conceive, and perhaps limit costly and unnecessary treatments.