The question of declining sperm density revisited: An analysis of 101 studies published 1934-1996

In 1992 Carlsen et al. reported a significant global decline in sperm densi ty between 1938 and 1990 [Evidence for Decreasing Quality of Semen during L ast 50 Years. Br Med J 305:609-613 (1992)]. We subsequently published a rea nalysis of the studies included by Carlsen et al. [Swan et at. Have Sperm D ensities Declined? A Reanalysis of Global Trend Data. Environ Health Perspe ct 105:1228-1232 (1997)]. In that analysis we found significant declines in sperm density in the United States and Europe/Australia after controlling for abstinence time, age, percent of men with proven fertility, and specime n collection method. The declines in sperm density in the United States (ap proximately 1.5%/year) and Europe/Australia (approximately 3%/year) were so mewhat greater than the average decline reported by Carlsen et al. (approxi mately 1%/year). However, we found no decline in sperm density in non-Weste rn countries, for which data were very limited. In the current study, we us ed similar methods to analyze an expanded set of studies. We added 47 Engli sh language studies published in 1934-1996 to those we had analyzed previou sly. The average decline in sperm count was virtually unchanged from that r eported previously by Carlsen et al. (slope = (-)0.94 vs. (-)0.93). The slo pes in the three geographic groupings were also similar to those we reporte d earlier. In North America, the slope was somewhat less than the slope we had found for the United States (slope = (-)0.80; 95% confidence interval ( CI), (-)1.37-(-)0.24). Similarly, the decline in Europe (slope = (-)2.35; C I, (-)3.66-(-)1.05) was somewhat less than reported previously. As before, studies from other countries showed no trend (slope = (-)0.21; CI, (-)2.30- 1.88). These results are consistent with those of Carlsen et al. and our pr evious results, suggesting that the reported trends are not dependent on th e particular studies included by Carlsen et al. and that the observed trend s previously reported for 1938-1990 are also seen in data from 1934-1996.