The headline on yesterday's story about seasonal changes in sperm count incorrectly said the phenomenon in question was potency. The study did not deal with potency. (Published 7/6/90)

By studying semen from men who work outdoors in Texas, a group of scientists has confirmed a long-held suspicion that the concentration of sperm drops during the summer and increases during the winter.

The report, in today's New England Journal of Medicine, may explain why the number of births decreases in the spring, researchers say. It may also have important implications for couples trying to conceive through methods such as artificial insemination and in vitro fertilization.

"It may be easier for a man to impregnate a woman in the wintertime -- or, more to the point, it's more difficult to impregnate a woman in the summertime," said Richard Levine, head of the epidemiology department at the Chemical Industry Institute of Technology in Research Park, N.C., and an author of the study. "If a person is having difficulty having children, perhaps it would be better to go at it full force in the wintertime."

The study also found that the drop in sperm count was unrelated to temperature -- leading researchers to speculate that perhaps an internal biological clock is responsible for the fluctuation.

"It's a real biological phenomenon," Levine said. "We have just put the icing on the cake by doing a real definitive study."

The study looked at semen from 131 men, all of whom worked outdoors for at least four hours daily during the summer. The semen was videotaped under a microscope. A computer then analyzed the sperm for motility and computed the concentration of sperm in the semen.

Samples of semen were taken in August 1986 and February 1987 in San Antonio. The average high temperature for July and August was 97 degrees Fahrenheit; in January and February it was 64.

The researchers found that the concentration of sperm in semen dropped 29 percent, on average, in the summer. They also found that the concentration of motile sperm -- sperm thought to be able to swim to the egg -- was lower in summer than in winter.

One longstanding theory had been that sperm concentration drops in summer because of the heat. But when the researchers compared semen from men who worked outside for only four hours daily with that from men who worked outside for as much as 12 hours, they found no differences. They had expected that if heat were a cause, longer exposure would have had a greater effect.

This was not the first study to examine seasonal differences in semen. But most of the others looked at past data from men in infertility clinics. No other such study has taken such a large group of men and followed them over time, Levine said.

Levine said the study helps solve the mystery of why the birth rate peaks in the late summer and early fall and drops in the spring. In 1987 -- the most recent year for which data are available -- the number of births in the United States ranged from a high of 336,381 in July to a low of 283,477 in February. Other studies, in Bangladesh and Indiana, found the frequency of sexual intercourse did not differ significantly between summer and winter, which led Levine and others to conclude that the difference is due to biological, and not behavioral, seasonal changes.

Levine said more studies are needed to determine whether the difference is due to day length or to inborn genetic factors.

But Levine and others said the study's most important implication may be for couples trying to conceive through artificial methods. The cost of in vitro fertilization, in which sperm is put in a dish containing an egg, can range from $6,000 to $9,000 per try, according to the American Fertility Society, a reproductive-education group. Many couples make more than one try. The cost of artificial insemination, in which sperm is injected into the woman's cervix, can range from $40 to $200 when the husband is the sperm donor, the society said.

Fertility specialists said the study's results are also important for drug companies working on infertility drugs for men and for researchers studying whether exposure to a chemical -- in a factory, for example -- affects male fertility.