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Authors Levine H, Jørgensen N, Martino-Andrade A, et al.
Review Date July 2017
Citation Human Reproduction Update 2017: 10.1093/humupd/dmx022 (online ahead of print)
For many years there have been reports of declines in sperm counts around the world. However, these claims are still controversial due to the quality of the studies and changes in sperm count methods over time. As the question of declining sperm counts is important given the implications for male fertility and the potential for intervention with respect to environmental influences, the authors have conducted a systematic review and meta-analysis of relevant research.
To conduct a systematic review and meta-regression analysis of recent (1981 to 2013) trends in sperm counts as measured by sperm concentration (SC) and total sperm count (TSC), and the effects of fertility status and geographic area on these trends.
PubMed/MEDLINE and EMBASE were searched for English language studies of human SC published between 1981 and 2013. The main MeSH term used was ‘sperm count’ (with 7 additional terms) as well as 13 related key words. Following a predefined protocol 7518 abstracts were screened and 2510 full articles reporting primary data on SC were reviewed. Studies were divided based on fertility status (men unselected for fertility status; fertile men, that is those known to have conceived a pregnancy) and studies were excluded if the participants were selected for infertility or sub-fertility, or for specific exposures known to affect fertility. Studies with fewer than 10 men and those not using standard methods to collect or count sperm were also excluded.
A total of 244 estimates of SC and TSC from 185 studies of 42,935 men who provided semen samples between 1973 and 2011 were extracted for meta-regression analysis, as well as information on years of sample collection and covariates: fertility status (‘unselected by fertility’ vs ‘fertile’), geographic area (‘western’ [North America, Europe, Australia, New Zealand] versus ‘other’ [South America, Asia and Africa]), age, ejaculation abstinence time, semen collection method, method of measuring SC and semen volume, exclusion criteria and indicators of completeness of covariate data.
The slopes of SC and TSC were estimated as functions of sample collection year using both simple linear regression and weighted meta-regression models and the latter were adjusted for pre-determined covariates and modification by fertility status and geographic area. Assumptions were examined using multiple sensitivity analyses and nonlinear models.
SC declined significantly between 1973 and 2011 in both unadjusted simple regression model (slope = −0.70 million/mL/year; 95% CI: −0.72 to −0.69; p < 0.001) and in adjusted meta-regression models (slope = −0.64 mill/mL/year; −1.06 to −0.22; p = 0.003). The slopes in the meta-regression model were modified by fertility (interaction: p = 0.064) and geographic group (interaction: p = 0.027).
There was a significant decline in SC between 1973 and 2011 among unselected by fertility, western populations (slope = −1.38 mill/mL/yr; −2.02 to −0.74; p < 0.001) and among fertile, western populations (slope = −0.68 mill/mL/yr; −1.31 to −0.05; p = 0.033), while no significant trends were seen among both unselected and fertile, ‘other’ geographic area populations.
Among unselected, western studies, the mean change in SC was -1.4% per year with an overall decline of 52.4% between 1973 and 2011. Trends for TSC were similar to SC, with a steep decline among unselected, western populations (−5.33 mill/year, −7.56 to −3.11; p < 0.001), corresponding to mean yearly decline in TSC of 1.6% and overall decline of 59.3%.
Multiple sensitivity analyses showed no significant effect on results, and there was no statistical support for the use of a nonlinear model. In a model restricted to data post-1995, the slope both for SC and TSC among unselected, western populations was similar to that for the entire period (−2.06 mill/mL, −3.38 to −0.74; p = 0.004 and −8.12 million, −13.73 to −2.51, p = 0.006, respectively).
This comprehensive meta-regression analysis showed a significant decline in sperm counts (as measured by SC and TSC) between 1973 and 2011, with a 50 to 60% decline among men unselected by fertility from North America, Europe, Australia and New Zealand (western populations). Because of the significant public health implications of declining sperm counts, there needs to be a focus on the possible causes of the observed declines in sperm counts and potential interventions to arrest this trend.
Points to Note
- To attempt to settle the question of declining sperm counts, this paper provides the best estimate to date of trends in sperm counts over time, by doing a meta-analysis of the many studies from recent decades.
- The reported trend of 50-60% decline in sperm counts in western populations over 40 years needs to be considered in the light of the limitations of the individual studies in the meta-analysis and the fact that results have been inconsistent – some studies have actually shown an increase in sperm counts and some no change over time.
- Other measures of sperm quality such as motility and morphology were not included in this analysis because these measures were not available in the earlier studies.
- A decline in natural conception rates that might be expected to accompany declining sperm counts have not been reported so far.
- Importantly, the causes of declining sperm counts in western countries including possible prenatal factors (such as endocrine disruption from chemical exposure or maternal smoking) and postnatal factors (such as obesity, ‘lifestyle’ behaviours, pesticide/chemical exposure) need to be investigated and preventive strategies identified.