And for the first time, sperm counts have been found to vary greatly from year to year -- and those fluctuations are linked to the birth rate in the general population, according to the report in the March issue of the Journal of Urology.
Our data revealed that the overall linear trend was an increase in sperm counts from 1971 to 1994, wrote lead study author Dr. Harry Fisch, director of the Male Reproductive Center at Columbia-Presbyterian Medical Center in New York. However, sperm counts varied from year to year -- reaching an all time high in 1980 at an average of 123 million sperm per milliliter of semen and a low in 1974 -- when on average 46.5 million sperm were found per milliliter. The study included sperm samples from 660 men collected at a Roseville, Minnesota-based sperm bank. Fisch and colleagues found that this year-to-year variation correlated to the birth rate in both Minnesota and the U.S. in general.
It is possible that the changes in birth rates were influenced by changes in sperm counts, according to the authors. The reason for the roller-coaster sperm statistics? The results may be due to environmental temperatures, according to the report.
Why sperm counts fluctuate so dramatically from year to year is not known, but heat may have a significant role, Fisch wrote. It is well known that mild changes in scrotal temperature can decrease sperm counts.
Previous studies have shown that sperm counts fluctuate with season, reaching a peak in the winter and dropping in the summer. It is certainly possible that yearly temperature and climate changes may affect annual sperm counts in a similar manner as seasonal changes affect sperm counts, according to the New York researchers.
Previous studies have found that sperm counts have dropped over time, but those reports have been highly criticized because of a number of potential problems with the way the studies were conducted, including whether sperm was counted in a consistent manner.
SOURCE: The Journal of Urology (1997)