Among unselected western studies (in men who were "unselected by fertility" vs those who were "fertile"), the mean SC declined 1.4% a year, leading to a drop of 52.4% in the 38-year period. Trends for total SC (TSC) were similar, with an average drop in mean TSC of 1.6% a year, for a decline of 59.3% during the period.
There were no significant trends among the "unselected other" (unselected men from South America, Asia, and Africa) group and the "fertile other" (fertile men from South America, Asia, and Africa) group.
"Because of the significant public health implications of these results, research on the causes of this continuing decline is urgently needed," Hagai Levine, MD, MPH, from the Braun School of Public Health and Community Medicine at Hadassah-Hebrew University in Jerusalem, Israel, and colleagues, write.
The findings are published online July 25 in Human Reproduction Update .
The decline means an increasing proportion of men in western countries are dropping to subfertile or infertile classifications.
"The high proportion of men from western countries with concentration below 40 million/ml is particularly concerning given the evidence that SC below this threshold is associated with a decreased monthly probability of conception," the researchers write.
These slopes held true after adjustment for such factors as age, ejaculation abstinence time, methods of collecting semen and counting sperm, selection of population, and exclusion criteria and in multiple sensitivity analyses, the authors write.
The meta-regression analysis considered 185 studies, on the basis of samples collected between 1973 and 2011 from 42,935 men. Data came from 6 continents and 50 countries.
Previous studies have gone back as far as 1931 and have been criticized for being less reliable, with the chance of historical measurement error.
The average SC in this study was 81 million/mL, and the mean TSC was 260 million/mL.
The decline did not show a leveling off when researchers limited analyses to studies with sample collection from 1996 to 2011.
When the authors considered only data for years after 1985, the slope "was similar to that for the full model," they write.
However, when they considered only data for years after 1995, the slope of decline in the unselected western group was "somewhat steeper" for both SC and TSC (−2.06 million/mL [95% confidence interval (CI), −3.38 to −0.74 million/mL; P = .004] and −8.12 million/mL [95% CI, −13.73 to −2.51 million/mL; P = .006], respectively), the authors write.
"Canary in the Coal Mine"?
The findings have implications beyond fertility and are consistent with trends in testicular cancer, cryptorchidism, testosterone levels, and onset of male puberty, the authors note. Low sperm count has also been connected to overall morbidity and risk for death.
The results may also represent a "canary in the coal mine," the authors say. Although the study was not designed to determine the cause of the decline, sperm count has been linked with environmental and lifestyle factors, including endocrine disruptors or mothers' smoking during a key window of male reproductive development.
Exposures to pesticides may also play a role in adult life, they note.
Study limitations include the exclusion of non–English-language publications, which may have limited the analyses of non-western countries.
Support was provided by the Environment and Health Fund, Jerusalem; American Healthcare Professionals and Friends for Medicine in Israel; Israel Medical Association; Research Fund of Rigshospitalet; the Brazilian National Council for Scientific and Technological Development; and the Mount Sinai Transdisciplinary Center on Early Environmental Exposures. The authors have disclosed no relevant financial relationships.
Human Reprod Update. Published online July 25, 2017. Abstract
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