In the 1970s and early 1980s several articles raised the possibility of a temporal deterioration in human semen quality1,3,70,71,72; however, these articles were not based on a true sequential analysis of semen. The first studies analysing semen data over time were published in the early 1980s and, since then, 87 articles have reported analysis of temporal trends in human semen quality. These studies used different designs and methodologies (Fig. 1): five were repeated cross-sectional studies, 68 were single-centre retrospective studies, five were multicentre retrospective studies based on individual data, and nine were multicentre retrospective studies based on mean, median or estimated values and not on individual data (Supplementary Table 2).

Most studies focused on the temporal evolution of sperm production (that is, sperm concentration and/or total sperm count), the assessment of which is, by nature, more objective than the assessment of sperm motility or sperm morphology. A much smaller number of studies also reported trends for other semen characteristics, including seminal volume, percentage of motile spermatozoa and morphologically normal spermatozoa.

Single-centre studies with repeated cross-sectional data

Only five studies have examined temporal trends in semen quality using repeated cross-sectional data, four in military conscripts from Scandinavia supposed to represent the general population and one in US male partners from infertile couples60,73,74,75,76 (Supplementary Table 2).

A Swedish study73 in 295 young men (age 17–20 years; median 18 years) born and raised in Sweden assessed men being recruited to military service. The participants delivered an ejaculate during 2008–2010 and their semen characteristics were compared with those of a similar cohort of Swedish military recruits aged ~18 years (n = 216) recruited in 2000–2001. Linear regression analyses estimated mean differences with 95% confidence intervals (CIs) between cohorts A and B with abstinence time (five categories), smoking status and BMI included in the models as potential confounders. No significant changes were found between 2000–2001 data and 2008–2010 data in sperm concentration (78 × 106/ml versus 82 × 106/ml; P = 0.54), semen volume (3.1 ml versus 3.0 ml; P = 0.26) or total sperm count (220 × 106 versus 250 × 106; P = 0.18). The proportion of progressively motile spermatozoa also remained unchanged.

A separate Finnish study74 in 858 volunteer young men (participation rate 13.4%) during 1998–2006 examined temporal trends in semen quality, using lists of Finnish young men who were required to attend a medical examination when they were 18–19 years old, irrespective of whether they were fit for military service. Participants had to live in the Turku area and their mothers had to be born in Finland. Semen samples were assessed using standardized methodology by a single technician across all study years. Temporal trends according to investigation period or birth cohort were tested by linear regressions adjusted for several confounders. Results showed a decrease in sperm concentration, total sperm count and percentage normal morphology compared with earlier time periods (P = 0.02, P = 0.03 and P = 0.03, respectively).

A similar cross-sectional study75 examined temporal changes in semen data in 4,867 Danish military conscripts with a median age of 19 years from 1996 to 2010. Inclusion criteria were place of residence in the Copenhagen area and that both the man and his mother had to be born and raised in Denmark. Seminal volume, sperm concentration, total sperm count, sperm motility and sperm morphology were assessed using a standardized method and study participants were divided into three groups according to the investigation period: 1996–2000, 2001–2005 and 2006–2010, with temporal trends tested by linear regression adjusted for confounders. Over the 15 years, median sperm concentration increased from 43 × 106/ml in 1996–2000 to 48 × 106/ml in 2006–2010 (P = 0.02) and total sperm count from 132 × 106 to 151 × 106 (P = 0.001) in the same periods. The median percentage of motile spermatozoa and abnormal spermatozoa were 68% and 93%, respectively, and did not change during the study period. However, the authors highlighted that the seminal volume, sperm concentration, total sperm count, total number of morphologically normal spermatozoa and percentage of normal spermatozoa were all lower in this group of young men (all P < 0.0005), than in a previously examined group of 349 fertile Danish men who had a median age of 31 years42.

This Danish study was later extended to total of >6,000 young Danish men recruited using the same protocol during a 21-year study period (1996–2016)60 and the same population of military conscripts following the same methodology. Overall, no major changes were seen in adjusted semen data except for percentage motility, which significantly increased (P < 0.001) between 1996 and 2016. Differences in semen parameters over the study period were small and similar in unadjusted and adjusted models.

A US cross-sectional study76 included men aged 18–56 years from couples seeking infertility treatment at the Massachusetts General Hospital between 2000 and 2017. The primary aim of the analysis was to identify environmental determinants of fertility, but the design of the study enabled examination of temporal trends in semen quality. Of note, semen quality did not differ between men who ultimately enrolled in the study and those who did not. The final study sample included 936 men who provided a total of 1,618 semen samples, and sperm concentration and motility were assessed using a computer-aided semen analyser with quality control and monitoring from andrologists who were trained in semen analysis. A multivariable generalized linear mixed model was used to estimate the differences in semen parameters, adjusting for abstinence time. Sperm concentration, total sperm count, percentage motility and percentage morphologically normal sperm decreased significantly over the study period: sperm concentration and total sperm count declined by 2.6% per year and 3.1% per year, respectively, corresponding to an overall decline of 37% and 42%, respectively, between 2000 and 2017. Trends towards a decrease were also observed for percentage motility and morphologically normal spermatozoa, with percentage declines of 15% and 16%, respectively, over the 17-year study period. Seminal volume remained stable over the study period. Of note, this particular study in male partners from infertile couples differed from other studies in the field as it included certain physical and reproductive factors as well as a set of data on environmental exposure parameters such as urinary concentrations of bisphenol A, parabens and phthalates. Interestingly, the negative temporal trends were found to be attenuated when examining the simultaneous changes in reproductive characteristics and urinary phthalates during the study, but, unfortunately, the lack of data on all potential predictors in all study participants during the study period prevented simultaneous evaluation of the possible combined role of all potential contributors to semen quality trends.

Strengths and limitations of these studies

The principal strength of most of these repeated cross-sectional studies is their study population; most are based on young military recruits. In practice, these studies benefited from access to lists of young men invited to undergo a physical examination for military service, who are considered to represent the general population. However, participation rates of such men in these studies are quite low (typically <25%), as is frequently the case for volunteers. Thus, the question arises as to whether these populations can still be considered representative of the general population54,61,77. Volunteer military conscripts are unlikely to have prior knowledge of their fertility potential, which means that this is unlikely to be the main determinant for their participation, which is essential to avoid introducing selection bias. Assessment of testosterone levels in men who agree or refuse to voluntarily give a semen sample has been proposed as a mean of assessing possible participation bias78, but similar concentrations of testosterone have been reported in volunteer military recruits agreeing or refusing to give a semen sample, suggesting no or minimal participation bias79. Of note, volunteers for research studies are often slightly better educated than those who do not volunteer80; although educational level itself is not directly relevant to sperm parameters, it can be related to other factors such as diet or smoking, that are likely to be relevant to sperm parameters.

The participation rates in studies of partners of pregnant women or occupational studies (32–54%21,81,82,83) are often higher than those reported for military recruits (typically <20%). For some studies, high participation rates might, at least partly, result from the home collection of semen samples, which are known to be of better quality than samples collected in a clinic or a laboratory56,57,58. In partners of pregnant women, agreement to provide a semen sample is not associated with age, socio-professional status, TTP, financial compensation or history of urogenital disease52. However, the possibility that social or reproductive history or sexual behaviour influences participation cannot be excluded.

Single-centre studies with retrospective data from individuals

Overall, 68 retrospective single-centre studies, using data from individuals, have been published across various geographical areas. Historically, most studies have come from from the Western world (41% Europe, 13% North America), but subsequently, studies have been published from other parts of the world (21% Asia, 9% South America, 7% Middle East, 7% Oceania and 1% North Africa), although data are still lacking for Russia and Sub-Sahelian Africa.

North America

To date, nine single-centre retrospective studies using data from individuals have examined temporal trends in semen quality in the USA: three in sperm donation candidates with unknown fertility status, five in mixed populations and one in infertile men.

The first, pioneering study in the field2 was published in 1981 and examined temporal trends in semen quality from US sperm donors of unknown fertility status during 1973–1980. All potential and accepted donors were requested to collect a semen sample after 3 days of sexual abstinence, and samples were assessed using a standardized method throughout the study period. The study reported a temporal decrease in sperm concentration when comparing data from 1977–1980 with data from before 1977 (P < 0.05). By contrast, percentage motility remained remarkably constant over the years. Percentage of normal spermatozoa decreased significantly from 1977 to 1980 (P < 0.05). However, the trends reported in this study were questionable, as the study design mixed intra-individual and inter-individual data from accepted and rejected donors over the study period.

A subsequent study in potential sperm donors of unknown fertility status from Wisconsin over a 10-year period (1978–1987)84 failed to detect any significant change over time for sperm concentration and percentage motility but the methodology of this study was poorly described, rendering the data unreliable. However, a separate study of 1,283 men who banked sperm before vasectomy in US sperm banks in Roseville, New York and, Los Angeles over a 25-year period (1970 to 1994)44 showed a slight, but significant, increase in mean sperm concentration for the total population (P = 0.04) as well as by individual centre in New York (r = 0.15, P = 0.002) and Roseville (r = 0.11, P = 0.006) but not in Los Angeles (r = 0.003, P = 0.06) after controlling for age and abstinence. No change in motility and a slight decrease in seminal volume (r = −0.07, P = 0.001) were found for the total population of the three centres over the 25-year period.

Semen data from 510 healthy adult men in Seattle and Tacoma, Washington area, were analysed between 1972 and 1993 (ref.85). Sperm concentration was measured by Coulter counter with a validated method and serial samples were collected from each individual, usually at 2-week intervals. Linear regression of mean sperm concentrations indicated a slight increase with time (P = 0.014) as well as slight, but significant, increases in seminal volume, total sperm count and percentage of normal spermatozoa.

Similarly, a retrospective study of 551 semen analysis records reported trends in semen characteristics in New England from 1972 to 1993 (ref.86). After age adjustment, sperm concentration showed a small upward trend of 0.2 × 106/ml per year (P < 0.01), and the authors also reported a 2.3% per year increase in percentage sperm motility and a 0.3% per year decrease in morphologically normal spermatozoa; however, no P values were reported.

By contrast, a decrease in sperm concentration was reported in a study of semen data for all men who applied to be a sperm donor in the Boston metropolitan area during 2003–2013 (ref.87). A total of 489 young adult men and 9,425 specimens were included in the analysis; specimens were collected by masturbation in a private room at the facility and were analysed using a standardized methodology. A general linear mixed model was used to evaluate the yearly trends, showing a statistically significant decrease in sperm concentration (−3.6 × 106/ml per year; 95% CI −4.9 to −2.2; P < 0.001) and percentage sperm motility (−11 × 106 per year; 95% CI −16.0 to −5.5; P < 0.001), as well as a significant decrease in percentage motility of −1.2% per year (95% CI −1.7 to −0.8). According to the individual’s year of birth, the P trend and β (95% CI) demonstrated a statistically significant decline in sperm concentration, P < 0.0001, 95% CI −1.1 (−1.6 to −0.7); total sperm count, P = 0.0008, 95% CI −3.6 (−5.7 to −1.5); and motility, P = 0.005, 95% CI −0.2 (−0.4 to −0.07), suggesting a possible decrease in sperm quality in association with both birth cohort and time period.

The temporal trend in total motile sperm count (TMSC) was evaluated using semen analyses of 119,972 subfertile men who presented to selected infertility centres in New Jersey in the USA and Valencia in Spain between 2002 and 2017 (ref.88). Semen analyses were categorized into three clinically relevant groups — group 1: TMSC >15 × 106; group 2: TMSC 5–15 × 106; and group 3: TMSC <5 × 106 — and relationships between male age, TMSC, trend and TMSC group by year were assessed. Overall, the proportion of men in group 1 was found to have declined approximately 10% over the past 16 years in the analysis that combined data from both centres. Although the choice to separate men into three groups is questionable, the authors acknowledged that several unknown factors might have influenced the findings.

South America

The first published study on semen trends in a non-Western country came from Venezuela89. Semen volume and sperm concentrations of 2,313 men from infertile couples from Merida between 1981 and 1995 were categorized in four groups according to sperm count. The frequency of azoospermia and oligozoospermia did not change over the 15 years of study. However, when an analysis of mean sperm concentrations was made in each group separately, a significant decrease was seen in men with high sperm counts (>200 × 106/ml) (P < 0.05) and a significant increase in men with sperm counts between 20 and 200 × 106/ml (P < 0.01).

Excluding the azoospermic group, the analysis of pooled data did not show a significant change in the mean sperm concentration through time.

A study in Sao Paulo, Brazil, analysed semen data from 182 sperm donors during 1992–2003 (ref.90). Semen analyses were performed by the same three laboratory technicians during the whole 10-year period, and the same laboratory methods were used to perform the semen analysis. Using multiple linear regression to evaluate the relationship between the year of semen collection and each seminal parameter controlling for potential confounders, sperm concentration was found to decrease (P < 0.0001) as did percentage normal sperm morphology (P < 0.0001) regardless of whether the semen sample analysed was the first or second donated sample. The seminal volume showed a slight increase (P = 0.038), whereas percentage motility did not change (P = 0.38). A second study in Sao Paulo91 reviewed semen data from 2,300 male partners from subfertile couples attending an assisted fertilization centre during 2000–2002 (n = 764) and 2010–2012 (n = 1,536). In this study, mean sperm concentration decreased significantly from 62 × 106/ml in 2000–2002 to 27 × 106/ml in 2010–2012 (P < 0.001). Mean total sperm count also decreased significantly over the same time period from 183 × 106 to 83 × 106 (P < 0.001) as did the mean percentage of morphologically normal spermatozoa, from 4.6% to 2.7% (P < 0.001). In addition, the incidence of severe oligozoospermia and azoospermia significantly increased from 16% to 30% (P < 0.001) and 4.9% to 8.5% (P = 0.001), respectively.

Also in Sao Paolo, semen data from 23,504 infertile men were evaluated over 7.5 years from 2010 to 2017 according to WHO 2010 guidelines17,92. A decreasing trend of 0.05 ml in seminal volume was observed over the period, alongside a tendency towards reduction in sperm concentration by 1 × 106/ml over the 7.5 years (mean of 34.3 × 106/ml). Over the entire study period, percentage sperm motility decreased by 0.7% (mean, 47.3%) and the percentage of morphologically normal spermatozoa decreased by 0.33% (mean, 2.8%), although no P values were reported.

A 2020 study reported temporal trends in semen characteristics in men admitted for infertility testing between 1995 and 2018 at Campinas University93. Only the first semen sample collected for each man was analysed (n = 9267), and the data were analysed using linear regression for the median values. In line with the previous study, overall, a significant decrease in the motile total sperm count (−2.8 × 106 per year, P < 0.001) and median percentage of normal spermatozoa (−0.52% each year, P < 0.001) was observed.

A study of Uruguayan men collected semen data from 317 healthy sperm donor candidates in Montevideo between 1988 and 2019 (ref.94). Semen samples were obtained by masturbation after 3–5 days of sexual abstinence and analysed according to the WHO 1980 and 2010 guidelines13,17, before linear regression and multiple regression analyses were used to calculate changes in sperm concentration and total sperm count per year. Similarly to the Brazilian data, sperm concentration decreased significantly over the 30 years by 0.9 × 106/ml per year, but total sperm count was unchanged (P = 0.1194). A significant change was also seen in percentage normal morphology over the study duration, but the other semen characteristics remained unchanged.

Scandinavia

In 1984, a study in 185 men from Malmö, Sweden95 examined semen quality in 1980–1981 and compared these data with semen analyses of age-matched control men from 1960–1961. By comparison with the earlier data, mean seminal volume and sperm concentration decreased from 3.8 ml in 1960–1961 to 3.4 ml in 1980–1981 (P < 0.05), and from 125 × 106/ml to 78 × 106/ml (P < 0.001), respectively, suggesting a decrease in semen parameters over the 20 years between the sample collections.

A subsequent study from Stockholm96 compared temporal changes in semen data of partners in infertile couples recorded in 1956 (n = 141), 1966 (n = 201), 1976–1979 (n = 219) and 1986 (n = 224) excluding azoospermia samples. In accordance with the previous work, mean total sperm count decreased from 467 × 106 in 1956 to 305 × 106 in 1986 (P < 0.0001), and percentage of morphologically normal spermatozoa also decreased, from 53% in 1956 to 37% in 1986 (P < 0.0001).

Considering the 1985–1995 period, a study from Lund, Sweden97, investigated semen quality in 718 male partners of infertile couples. Time-related changes were analysed using linear regression. In contrast to the previous studies from Stockholm, this analysis showed a significant increase in mean sperm concentration from 46 × 106/ml in 1985 to 64 × 106/ml in 1995 (P < 0.001); mean percentage of morphologically normal spermatozoa also increased from 58% to 66% (P < 0.001). Mean total sperm count did not change, whereas mean seminal volume decreased significantly during this period, from 3.6 ml to 2.7 ml (P = 0.002).

A study of 5,481 Finnish men from infertile couples in Turku98 examined changes in sperm count during a 28-year period, 1967–1994. Mean semen volume, sperm concentration and total sperm count in normal men were 3.3 ml, 134 × 106/ml and 397 × 106, respectively; multiple linear regression analysis revealed a significant decrease in semen volume (P < 0.001), whereas sperm concentration and total sperm count did not change. Of note, no change in sperm count was associated with the men’s year of birth.

In Denmark, a study of 1,055 men born between 1950 and 1970 in Odense99 reviewed semen data at the time of their female partner’s first IVF cycle between 1990 and 1996. These men were assumed to represent a random sample of the Danish male population of fertile age. Semen analyses were performed by the same six technicians using the same counting chambers throughout the study period, minimizing both the intra-assay and inter-assay variations. Mean sperm concentration was 183.7 × 106/ml and mean semen volume was 3.9 ml but, although considerable variation in both parameters was found from year to year, no significant change occurred in either parameter throughout the entire period. When men were stratified according to their birth year, a later year of birth was not associated with any change in sperm concentration or semen volume.

Another study from Denmark investigated whether semen quality changed between 1977 and 1995 in a group of 1,927 unselected semen donor candidates from Copenhagen47. Donors were recruited through advertisements in student periodicals and had to be between 18 and 35 years old, but no other selection criteria were specified. Multiple linear regression analysis using year, sexual abstinence and season as covariates, showed a significant increase in mean sperm concentration from 53 × 106/ml in 1977 to 72.7 × 106/ml in 1995 (P < 0.0001) and in mean total sperm count from 166 × 106 to 228 × 106 (P < 0.0001) and these data showed significant variation between seasons (P < 0.0001 for both parameters). However, the authors indicated that they were unable to control for variation in donor age and, therefore, cannot exclude the possibility of selection bias, whereby participants were accepted as donors by other semen donor services in Copenhagen.

In Norway, potential secular trends in semen characteristics from men of Bergen were assessed according to previous or subsequent paternity during the period 1975–1994 (ref.100). Samples were collected from men under investigation for infertility — 1,108 men who had fathered at least one child before the analysis, 1,786 men who had at least one child after the analysis and 2,286 men with no children registered. When analysed by year of evaluation, registered childless men had a significant decrease in sperm concentration (P < 0.015) and total sperm count (P < 0.001) over the study period. Likewise, the group with subsequent children had significant temporal decrease in sperm concentration (P < 0.015) and total sperm count (P < 0.047), whereas no significant changes were found for the group with previous children. Analysed by year of birth, a significant decrease in sperm concentration (P < 0.025) and total sperm count (P < 0.003) was found for the childless group and for the group with subsequent children (P = 0.012 and P = 0.015, respectively). Otherwise, no significant trends were found.

In a separate study, semen analysis records were studied for all men (n = 5,739) who attended the fertility clinic of Tromsø from 1993 to 2012 (ref.101). Semen samples from men who all resided in the Northern region of Norway were assessed following WHO 1987, 1992 and 2010 recommendations14,15,17. Using multiple regression models accounting for the effect of men’s age and calendar year on semen characteristics, a gradually decreasing trend of mean total sperm count per ejaculate was observed during the study period (P < 0.001), and mean sperm concentration and seminal volume were also found to significantly decrease.

Germany

The first German study to assess sperm parameters102 was published in 1997 and included 187 young male volunteers from Munster, who were recruited via bulletin boards in universities and local newspapers. Samples were collected by masturbation after a requested period of abstinence ranging from 2 to 7 days and analysis was performed as recommended in the WHO manual (1980, 1987, 1992)13,14,15. In this study, no obvious trend over time was observed for sperm concentration, total sperm count or total motility.

By contrast, a subsequent study103 investigated mean sperm concentration and motility of 5,149 men in Magdeburg from 1974 to 1994. The laboratory methods used and the criteria applied to analyse sperm count and motility did not change during this 20-year period and participants were not preselected. Between 1974 and 1976 the mean sperm concentration was 48 × l06/ml, decreasing by 2.1% per year to 26 × 106/ml between 1992 and 1994 (P < 0.001). Likewise, the mean percentage of motile spermatozoa decreased from 38% to 22% and the mean percentage of morphologically normal spermatozoa from 64% to 42% in the same period (both, P < 0.001).

A later study from Leipzig, which assessed characteristics of the first semen specimen obtained from 3,432 patients aged 24–35 years who had attended the Department of Andrology during 1975–2000, showed mixed temporal trends in sperm parameters104. Notably, the population studied was characterized by very low geographical mobility and relocation because of the social and political situation in East Germany. Semen analyses were performed using a standardized method that remained unchanged during the study period. No changes in sperm count or percentage motility were found when analysed by year of semen analysis or age at time of examination; however, by contrast, sperm concentration and total sperm count showed a negatively significant correlation with the year of birth between 1958 and 1968 (both, P < 0.01).

Scotland

A 1996 study of men in Scotland provided early evidence of deteriorating semen quality, using semen data from 577 men from a sperm donation programme in Edinburgh between 1984 and 1995 (ref.63). All samples were analysed in one laboratory according to a standardized method, and relationships between variables were examined using linear and stepwise multiple linear regression. In addition, donors were divided into four roughly equal cohorts of 5 years according to year of birth. Ejaculate volume did not correlate with either year of birth or age at donation. By contrast, sperm concentration decreased by 2.1% per year and total sperm count by 2.0% per year. Overall, motility was weakly positively correlated with a later year of birth, increasing by 0.18% per year. No relationship was observed between the year of donation and any measures of semen quality except overall motility, which increased by 1.2% per year. The median sperm concentration (x106/ml) among donors born in the 1950s was 98, falling to 78 among those born in the 1970s (P = 0.002). The overall percentage of motile sperm did not show any change from the 1950s’ to the 1970s’ birth cohorts.

A second study in the northeast of Scotland — an area where migration rates are low and where andrology services for a population of 500,000 are centralized — examined population-based trends in semen quality between 1994 and 2005 in a cohort of 4,832 men with a sperm concentration of >20 × 106/ml attending for routine semen analysis at the Aberdeen Fertility Centre105. Data adjusted for age and period of abstinence showed a decreasing trend in sperm concentration during the study period (P = 0.017), but no such trend was seen in sperm motility or motile density (total count of motile spermatozoa (millions/ejaculate)). The authors indicated that this trend should be interpreted with caution owing to fluctuations in semen parameters, population bias and the retrospective nature of the analysis.

Belgium

A 1996 study reviewed semen data from 416 candidate sperm donors at University Hospital, Ghent during 1977–1995 (ref.106). The men were recruited through advertising in local journals and student periodicals and most were students or paramedical personnel who had not fathered any children. Semen was analysed using conventional techniques described in the WHO Laboratory Manual (1987)14, and most analyses were performed by the same technician using the same method. A slight, but not significant, decrease in sperm concentration was observed (P = 0.08), alongside a slight linear increase but not significant in ejaculate volume with time (P < 0.06), whereas the total sperm count did not change. Percentage sperm motility and percentage normal morphology were found to significantly decrease (respectively, r = −0.42, P < 0.0001 and r = −0.23, P < 0.0001).

A 2021 study analysed semen data from 439 candidate donors in Antwerp over a 23-year period (1995–2017)107. Over the entire study period, a temporal decrease was observed only for normal sperm morphology (P < 0.0001), whereas all other parameters remained largely unchanged. The mean clinical pregnancy rate per effective donor recruited (n = 104) did not change according to year of donation, as the donors recruited had normal sperm parameters.

France

A 1995 study analysed 1,351 candidate semen donors in a Parisian university sperm bank between 1973 and 1992 to investigate a possible temporal trend108. The men were all healthy, unpaid volunteers who had previously fathered at least one child and all samples were assessed following a standardized methodology. Ejaculate volume did not change during the study period. However, sperm concentration decreased by 2.1% per year, from 89 × 106/ml in 1973 to 60 × 106/ml in 1992. The percentages of motile and normal spermatozoa decreased by 0.6% and 0.5% per year, respectively (P < 0.001 for both). In addition, multiple regression analyses after adjustment for age and sexual abstinence revealed a 2.6% yearly decline in sperm concentration and a 0.3% and 0.7% yearly decline in the percentages of motile and normal spermatozoa, respectively, associated with each successive calendar year of birth (all P < 0.001).

A separate study that used similar recruitment modalities and procedures for semen assessment was subsequently carried out in Toulouse109. The study assessed first ejaculate from 302 fertile candidates for sperm donation whose semen was collected between 1977 and 1992. Linear regression analysis between sperm count and year of donation adjusted on donor’s age did not reveal any changes in this variable (r = 0.09, P > 0.05).

Another French study also reported the results of temporal trends in semen in 1,114 fertile men candidates for sperm donation from the sperm bank in Tours between 1976 and 2009 (ref.110). Only the first semen sample was taken into account, and semen was assessed according to 1980, 1987, 1992 and 1999 WHO guidelines13,14,