Importance: Temporal global trends of sperm quality remain a matter of debate.
Objective: The aim of this study was to present a comprehensive review of studies reporting on sperm quality counts, summarize the main end points, and assess the main reasons for potential discrepancies.
Evidence acquisition: An evidence-based review of PubMed and Scopus databases was performed regarding studies reporting on modification of sperm quality counts, independently of study character, study language, or date.
Results: Since the meta-analysis of Carlsen et al in 1992 (Br Med J 1992;305:609-613) that suggested an annual decline in sperm count of 1%, several reports confirmed the decline in sperm quality, whereas others disproved them, suggesting a slight increase or absence of change in sperm count. Such controversies may be attributed to geographical and time-related variability in sperm values and also to several confounding factors that influence the semen parameters. Intrinsic weaknesses of the studies include heterogeneity of subjects recruited, lack of adjustment for confounding factors, and samples that do not always represent the general population.
Conclusions: No consensus exists on whether sperm counts actually decrease because studies' results are often controversial or inconclusive with methodological deficiencies. More prospective, large-scale, population based studies are needed in order to provide sound evidence of possible global trends in sperm count.