Secular trends in semen parameters among men attending a fertility center between 2000 and 2017: Identifying potential predictors

Abstract

Background: Multiple meta-analyses have shown sperm count declines in Western countries spanning eight decades. Secular trends in other parameters remain unclear, as are potential predictors of these trends.

Objective: To analyze secular trends in semen quality and to evaluate whether factors previously found to be related to semen quality were responsible for these patterns.

Methods: This is a prospective study including 936 men of couples seeking infertility treatment who provided 1618 semen samples at a single center (2000-2017). Self-reported demographic, nutritional and reproductive characteristics were collected using standardized questionnaires. Urinary concentrations of bisphenol A, parabens and phthalates were quantified by isotope-dilution tandem mass spectrometry. Semen samples were analyzed for volume, sperm concentration, count, motility and morphology following WHO guidelines. We estimated the differences in semen parameters over time by fitting generalized linear mixed models with random intercepts to account for repeated samples while adjusting for abstinence time. We also adjusted for demographic, nutritional and environmental factors to investigate these as potential predictors of time trends.

Results: Sperm concentration and count declined by 2.62% per year (95% CI -3.84, -1.38) and 3.12% per year (95% CI: -4.42, -1.80), corresponding to an overall decline of 37% and 42%, respectively, between 2000 and 2017. Decreasing trends were also observed for total motility (per year: -0.44 percentage units, 95% CI -0.71, -0.17) and morphologically normal sperm (per year: -0.069 percentage units, 95% CI -0.116, -0.023). These decreases reflected relative percentage declines of 15% and 16% over the 17 year study period, respectively. When reproductive factors were included in the model, the downward trends in sperm concentration and sperm count were attenuated by 29% and 26%, respectively, while the trends in motility and morphology were attenuated by 54% and 53%, respectively. Also, the downward trends in both sperm concentration and sperm morphology over time were attenuated by 19% when including the DEHP and non-DEHP metabolites, respectively.

Conclusions: Sperm concentration, total count, motility and morphology significantly declined between 2000 and 2017 among subfertile men. These negative trends were attenuated when considering simultaneous changes in reproductive characteristics and urinary phthalates during the course of the study.

Keywords: Male infertility; Phthalates; Predictors; Secular trends; Semen parameters.

Figure 1.. Secular trends in sperm concentration, total sperm count, sperm motility and normal morphology among men attending a fertility center in Boston between 2000 and 2017.

The solid lines represent the adjusted mean for semen parameters by year of sample collection, the dotted lines are the upper and lower 95% confidence intervals for the adjusted means. Adjusted means are presented for average abstinence time (2 days). N=936 men and 1,618 semen samples for all parameters except sperm morphology, which was assessed in 1,482 semen samples.