Male Fertility Drops During Summertime, Researchers Report

Sperm counts decrease during the summer, perhaps explaining the reduction in the birth rate during the spring in parts of the world with warm climates, according to a new study.

The findings, which are being published in today’s New England Journal of Medicine, may also help physicians treat male fertility problems.

“It is more difficult for a man to impregnate a woman during the summertime than some other season, particularly the winter,” said Dr. Richard J. Levine of the Chemical Industry Institute of Toxicology in Research Triangle Park, N. C.

Levine, leader of the research team, said that male infertility patients might get better results if specialized treatments, such as injections of sperm directly into the uterus, were offered in the winter months when “the man would have more sperm to work with.”

Dr. Peter J. Snyder of the University of Pennsylvania said that seasonal differences in sperm counts are probably not significant for men with normal fertility but might make a difference for men with borderline fertility. Snyder wrote an editorial for the medical journal about the study.

In interviews, Levine and Snyder acknowledged that researchers do not fully understand the seasonal variations in sperm count.

Levine and his colleagues compared semen specimens obtained in summer and winter from 131 normal men who worked outdoors near San Antonio, Tex.

When compared to winter, they found “significant reductions”--typically between one-quarter and one-third--during the summer in sperm concentration, total sperm count in each ejaculation and the concentration of motile sperm, those most likely to fertilize an egg.

The average sperm count decreased from 140.5 million sperm per milliliter of semen in the winter to 100.1 million sperm per milliliter in the summer. Both of these values are far higher than the lower limit of a “normal” sperm count, which is 20 million sperm per milliliter.

But the proportion of men at greatest risk of infertility--those with sperm concentrations of under 20 million sperm per milliliter--increased from 1 of 131, or less than 1%, in winter to 13 of 131, or about 10%, in summer.

The study “raises the question of whether people with borderline fertility may be affected in this manner,” said Dr. Jacob Rajfer, a urologist at the UCLA Medical Center who specializes in male infertility. But Rajfer cautioned that this question could not be answered until studies focusing on men with borderline sperm counts were performed.

The researchers also tracked the month of birth for 73 children of the men, whose wives had lived within 250 miles of San Antonio during the year before childbirth.

They found that about 29% of births occurred in the summer, 29% in the fall, 25% in the winter and only 17% occured in the spring. There was a 6% possibility that these differences were due to chance. Most babies born in the spring were conceived during the preceding summer.

In their analysis, the researchers accounted for other factors that can affect semen, such as sexual abstinence, which can increase the sperm count over the course of several days.

Snyder praised the study, saying that “while other people have thought of this before,” the new research “is just generally better done” and the results “are more believable.”