In warm climates throughout the world, there is a deficit of births during the spring season. To determine whether this deficit might reflect a deleterious effect of heat on the male reproductive capacity during the previous summer, we obtained semen specimens in summer and winter from normal men who worked outdoors in the vicinity of San Antonio, Texas, and we performed automated semen analyses with an image-analysis system. Pairwise comparisons among 131 men without azoospermia who contributed specimens in both summer and winter revealed significant reductions during the summer in sperm concentration, total sperm count per ejaculate, and concentration of motile sperm. The mean decreases in these values after adjustment for potential confounding characteristics were 32 percent (95 percent confidence limits, 28 and 44 percent), 24 percent (95 percent confidence limits, 18 and 43 percent), and 28 percent (95 percent confidence limits, 24 and 44 percent), respectively (P<0.0001). The lower a subject's average sperm concentration and motile-sperm concentration, the greater the reduction. We found no correlation, however, between the number of hours per day spent working during summer in settings without air conditioning and either the summer sperm concentration or the difference in concentration between summer and winter. Among the children of the men in the study whose wives were living near San Antonio during the year before they gave birth, a disproportionately low number were born during the spring. Semen quality deteriorates during the summer. This phenomenon may account at least in part for the reduction in the birth rate during the spring in regions with warm climates. Demographers have repeatedly noted reductions in the birth rate during spring in nonequatorial regions with warm climates.1 2 3 4 Could summer heat nine months earlier have caused the quality of semen to deteriorate and result in a diminished rate of conception? Whereas homeostatic mechanisms should afford the female reproductive organs maximal protection against temperature changes in the environment, the thermoregulatory capacity of the scrotum is limited.5 Even mild scrotal warming can temporarily diminish the number of sperm ejaculated after two to three weeks.6 A decreased frequency of sexual intercourse could explain a reduced rate of conception during the summer, but the available… © 1990, Massachusetts Medical Society. All rights reserved.
