Spatiotemporal trends in human semen quality

Abstract

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.

Fig. 1. Study designs examining spatial and temporal trends in human semen quality.

Several different study types have been used to analyse trends in sperm quality. Each of these is subject to benefits and drawbacks, for example, according to the type of data collected and the population included.

Fig. 2. Subcontinental temporal distributions of sperm concentration estimates.

Data are separated by colours according to subcontinental regions: USA (part a), Australia and New Zealand (part b), Scandinavia (part c), western Europe (part d), southern Europe (part e), eastern Europe (part f). Part g shows an amalgamated representation of temporal distribution including all data from the Western world, merging all subcontinental region data. The dotted lines in parts a, b, c and f link successive estimates from a single centre. The figure uses original data provided by Levine et al., with a correction of the mean values of sperm concentration reported in the Rubes et al. study, which was ~140 × 10⁶/ml instead of the ~40 × 10⁶/ml mentioned in the provided dataset.