Over the past four decades, studies of various designs have reported spatial and temporal trends in human semen quality. Several standardized-methodology studies in homogeneous populations that compare specific cities within a country or a continent provide clear evidence of geographical differences in sperm production, even over short distances within the same country. Human sperm production is widely believed to be declining over time, but evidence from the scientific literature is less clear. Studies based on repeated cross-sectional data from a single centre have shown mixed results. Among the numerous retrospective studies conducted in a single centre, only some included homogeneous groups of men and appropriate methods, and most of them suggest a temporal decrease in human sperm production in the geographical areas considered. Conclusions reporting temporal trends in sperm production that came from existing retrospective multicentre studies based on individual semen data and those using means, medians or estimates of sperm production are questionable, owing to intrinsic limitations in the studies performed. Regardless of study design, studies on the percentage of motile or morphologically normal spermatozoa are still limited by the inherent variability in assessment. Overall, available data do not enable us to conclude that human semen quality is deteriorating worldwide or in the Western world, but that a trend is observed in some specific areas. To understand these trends and contrasts in sperm and semen quality, prospective studies should be encouraged and combined with assessment of the male exposome.

在过去的四年里，各种设计的研究报告了人类精液质量的空间和时间趋势。在同质人群中进行的几项标准化方法研究比较了一个国家或一个大陆内的特定城市，提供了精子产生的地理差异的明确证据，即使是在同一国家内的短距离内也是如此。人们普遍认为人类精子的产生会随着时间的推移而下降，但来自科学文献的证据却不太清楚。基于来自单个中心的重复横截面数据的研究显示出不同的结果。在单个中心进行的众多回顾性研究中，只有一些包括同质的男性群体和适当的方法，其中大多数表明在所考虑的地理区域内人类精子产量暂时下降。现有的基于个体精液数据的回顾性多中心研究以及使用精子产生的平均值、中位数或估计值的回顾性多中心研究得出的报告精子产生时间趋势的结论是值得怀疑的，因为所进行的研究存在内在局限性。无论研究设计如何，对活动精子或形态正常精子百分比的研究仍然受到评估中固有的可变性的限制。总体而言，现有数据不能让我们得出结论，人类精液质量在世界范围内或在西方世界正在恶化，但在某些特定领域观察到了趋势。为了了解精子和精液质量的这些趋势和对比，应鼓励前瞻性研究并将其与男性暴露组的评估相结合。