| Have Sperm Densities Declined? A Reanalysis of Global Trend Data Shanna H. Swan, Eric P. Elkin, and Laura Fenster Reproductive Epidemiology Section, California Department of Health Services, Emeryville, CA 94608 USA Abstract
In 1992 a worldwide decline in sperm density was reported ; this was quickly followed by numerous critiques and editorials. Because of the public health importance of this finding, a detailed reanalysis of data from 61 studies was warranted to resolve these issues. Multiple linear regression models (controlling for abstinence time, age, percent proven fertility, specimen collection method, study goal and location) were used to examine regional differences and the interaction between region (United States, Europe, and non-Western countries) and year. Nonlinear models and residual confounding were also examined in these data. Using a linear model (adjusted R 2 = 0.80) , means and slopes differed significantly across regions ( p = 0.02) . Mean sperm densities were highest in Europe and lowest in non-Western countries. A decline in sperm density was seen in the United States (studies from 1938-1988 ; slope = - 1.50 ; 95% confidence interval (CI) , - 1.90- - 1.10) and Europe (1971-1990 ; slope = - 3.13 ; CI, - 4.96- - 1.30) , but not in non-Western countries (1978-1989 ; slope = 1.56 ; CI, - 1.00-4.12) . Results from nonlinear models (quadratic and spline) were similar. Thus, further analysis of these studies supports a significant decline in sperm density in the United States and Europe. Confounding and selection bias are unlikely to account for these results. However, some intraregional differences were as large as mean decline in sperm density between 1938 and 1990, and recent reports from Europe and the United States further support large interarea differences in sperm density. Identifying the cause(s) of these regional and temporal differences, whether environmental or other, is clearly warranted. Key words : epidemiology, multiple regression analysis, semen quality, sperm density, trend. Environ Health Perspect 105:1228-1232 (1997) .
Address correspondence to S.H. Swan, Reproductive Epidemiology Section, Environmental Health Investigations Branch, California Department of Health Services, 5900 Hollis Street, Suite E, Emeryville, CA 94608 USA. The authors thank Alan Hubbard for his contribution to the statistical analysis. Received 27 May 1997 ; accepted 29 July 1997.
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