Sperm concentration remains stable among fertile American men: a systematic review and meta-analysis

Abstract

ImportanceFindings from several high-profile meta-analyses have raised concerns about an ongoing global decline in sperm concentration and male fertility. However, these studies exhibit considerable heterogeneity in key variables including study population, methodology, fertility status, and geographic region. ObjectiveTo perform a systematic review and meta-analysis exploring temporal trends in sperm concentration among fertile men and men unselected for fertility status in the United States. Data sourcesA literature search performed in Scopus and PubMed databases for studies published between 1970-2023. Additional studies were included from citations of prior global meta-analyses and reviews evaluating temporal trends in sperm count.Study selection and synthesis: Studies were included if they presented original data on sperm concentration in U.S. men without known infertility from 1970 to 2023. Aggregate data were assessed across all study populations, with additional subgroup analyses stratified by fertility status and U.S. region. Main outcomesWeighted generalized linear models were generated to evaluate the association between mean sperm concentration and sample collection year. ResultsA total of 874 articles were screened, with 58 meeting the inclusion criteria. These represented 75 unique study populations totaling 11,787 men in the U.S. Across all study populations, no change in sperm concentration was observed between 1970-2018 in unadjusted models (β=0.14million/mL/year, p=0.42). When adjusting for U.S. region, no statistically significant decline in sperm concentration was seen. When adjusting for both region and fertility status, a modest annual decline was observed to meet statistical significance (β=-0.35million/ml/year, p=0.04). Of the 49 study populations reporting adequate data to determine mean total sperm count, there was a significant increase in total sperm count of 2.9 million/year between 1970 and 2018 (p=0.03). Subgroup analysis found no statistically significant change in mean sperm concentration among any U.S. census region or fertility status cohort. Conclusion and relevanceIn contrast to prior global studies, this analysis suggests no clinically significant decline in sperm concentration among confirmed fertile men and the general male U.S. population without known infertility. While these findings provide some reassurance against a widespread rapid decline, further studies are necessary to better understand this important topic.