Have sperm densities declined? A reanalysis of global trend data.
Level 3 - non-randomized controlled study
Meta-regression and reanalysis of historical observational studies
PubMed 9370524 · doi:10.1289/ehp.971051228
What was done
Multiple linear regression, quadratic, and spline models reanalyzed data from 61 studies published between 1938 and 1990 to evaluate worldwide trends in sperm density. The models controlled for potential confounders including abstinence time, age, percentage of proven fertility, specimen collection method, study goal, and geographic location, evaluating differences across three regions (United States, Europe, and non-Western countries) as well as region-by-year interactions.
What was found
In a linear model accounting for covariates (adjusted R2 = 0.80), baseline means and temporal slopes differed significantly across regions (p = 0.02). Significant declines in sperm density occurred in the United States (1938-1988; slope = -1.50; 95% CI, -1.90 to -1.10) and Europe (1971-1990; slope = -3.13; 95% CI, -4.96 to -1.30). No significant decline was detected in non-Western countries (1978-1989; slope = 1.56; 95% CI, -1.00 to 4.12). Mean sperm densities were highest in Europe and lowest in non-Western countries. Nonlinear models yielded similar trends.
Why it matters
This study provides statistical evidence that reported 20th-century declines in sperm density in Western nations were not solely artifacts of confounding by age, abstinence, or collection methods. It established that substantial regional variations exist and reinforced the need to identify underlying environmental or biological causes.
Limits
The analysis is restricted to study-level aggregate data from heterogeneous observational publications rather than individual-level longitudinal cohorts. Data from non-Western regions were sparse and covered a narrow timeframe (1978-1989). Large intraregional and interarea differences were present, matching the magnitude of the overall temporal declines, leaving open the potential influence of localized unmeasured factors.
Cited by
- supports A reanalysis of 61 studies controlling for smoking, obesity rates, and sperm counting methods confirmed the 50-year sperm count decline trend.