In this large, prospective and well-controlled study of semen quality of annual cohorts of young men from the general population, statistically significant increases in sperm concentration and total sperm counts over the past 15 years were detected. However, it is of concern that these men from the general population in the new millennium had significantly lower sperm concentrations and total sperm counts than recently examined fertile men and men of a historical cohort of male partners of infertile couples. Both sperm concentration and sperm morphology measures according to strict criteria are known to be informative semen measurements for discriminating between fertile and infertile men.41
Therefore, there is reason to be concerned about future fertility of young Danish men. Smaller cross-sectional studies of men from the general populations in other European countries have shown similar high frequencies of men with poor semen quality.38–40
Thus, poor semen quality seems to be a widespread phenomenon. This interpretation is in line with the high and increasing need of fertility treatment in Denmark.44
We have considered whether immaturity of the men (with a median age of 19 years) could account for the findings. However, a 4-year longitudinal follow-up with quarterly assessment of semen quality in a subgroup of more than 150 of the men showed no significant change over time in sperm concentration, total sperm count and sperm morphology, suggesting that immaturity does not explain our results.45
It is also possible that our results could be skewed by selection biases. However, during the early stage of our project, we carried out a study on blood samples from the majority of those men who did not volunteer to provide semen samples (N=195, participation rate 79%) and showed that their reproductive hormone levels including the spermatogenesis markers follicle-stimulating hormone and inhibin-b were very similar to those of the participants.14
This suggests that our results are not biased by selection. Furthermore, our results hold true in the subgroup of men without andrological events in their history as presented in appendix 1
. It is not likely that the detected temporal trend in sperm count is due to intraobserver or interobserver variations. Our laboratory technicians participated in a quality control study of assessment of sperm concentration, which did not indicate temporal changes in assessment levels. However, a longer observation period is needed to corroborate or refute such a positive tendency. Five observers did 97.6% of all morphology assessments, among which a single observer assessed 91% of samples in the first 5-year period, 14% in the second period and none in the last. One observer assessed 14% of samples from the second period, but none from the first or last period. This observer tended to obtain 1% lower values than other observers (detailed data not shown), which partly explains the lower number of morphologically normal spermatozoa in the second 5-year period. Assessment of semen volume was also controlled and corrected when needed. Effects of potential confounders of semen variables were investigated and accounted for in the statistical analyses. Increasing duration of abstinence up to approximately 96 h had an increasing influence on semen volume, sperm concentration and total sperm count, but no effect on motility or morphology, which is in agreement with our initial findings and with the results of other semen quality studies of men from Europe.38–40
From pilot studies in the middle of 1990s, we know that interobserver variation for motility assessment is of significant importance46
and difficult to eliminate. Our results on numbers of motile sperms should therefore be taken with some caution.
The definition of normal semen quality has varied over time. Seventy years ago, the Danish standard for normal sperm concentration set 60 million/ml as a lower cut-off level.13
However, the most recent WHO guidelines adhere to common clinical practice, where the ‘normal’ reference range is defined as the one that covers 95% of a population. The most recent WHO guidelines have reduced the reference limits for sperm concentration from 20 to 15 million/ml.36
Reference limits based on 95% of the population may be relevant in relation to certain clinical tests (eg, levels of sodium or potassium in serum) but are unsuitable for public health issues in which secular changes may affect the whole population (eg, obesity).47
For trend analyses, our data on semen quality of men examined during the past 15 years should therefore rather be compared with data from the previous generations of men. Unfortunately, historical data on semen quality of men from the general population do not exist. Other unique Danish semen data obtained by the pioneer of modern Danish andrology, Dr Richard Hammen, who studied semen quality of men 70 years ago, exists.13
His method for counting sperm concentration by the use of the Bürker-Türk haemocytometer was very similar to that used in our present investigations and in accordance with the current recommendations by the WHO,36
allowing for meaningful comparisons with our new data. Interestingly, sperm numbers among men in the Hammen study from the 1940s were significantly higher than those in the present study, despite the fact that the earlier sample was recruited among male partners in infertile couples. This actually corroborates that semen quality might have decreased temporarily as suggested by the meta-analysis by Carlsen et al.1
Whereas the historical data point to a temporal decrease in sperm concentration and total sperm counts, there is no such data to support a similar trend in the percentages of normal spermatozoa in each ejaculate. A trend may be difficult to detect because different criteria for normality have been applied during the years. In our study, we did not find any trend in sperm morphology despite a slight increase in sperm numbers. However, it is noteworthy that the median percentage of spermatozoa with normal morphology was as low as 6.5%. In contrast to our study, a decrease in the percentage of normal spermatozoa was recently described in a Finnish study, which also reported decreasing trends for sperm concentration and total sperm counts.40
Although only one spermatozoon is needed to fertilise an egg, several studies have shown that the fertilising ability diminishes if the sperm concentration is below 40–50 million/ml or if the percentage of normal spermatozoa is below 9%.48–51
Approximately 42% of the men from the general population had sperm concentrations below 40 million/ml and 66% <9% normal forms. More severe fertility problems may be present when sperm concentration is below 15 million/ml and the percentage of normal spermatozoa is <5%,41
which was the case for 15% and 35% of the men from the general Danish population, respectively. It is noteworthy that only 8% and 18% of a group of fertile men in a previous study of partners of pregnant women were below these ‘cut-off’ levels. Sperm concentration, total sperm count and percentage of normal spermatozoa were significantly lower in men from the general population in comparison to fertile men. Only 23% of men from the general population had the optimal sperm concentrations of more than 40 million/ml and more than 9% normal forms in comparison to 42% of the fertile men.
Both clinical practice and animal studies suggest an important role of sperm morphology for conception rates.41
Human in vitro fertilisation studies also suggest an important role of sperm morphology for fertilisation rates, which become significantly lower if the percentage of normal spermatozoa is below 5%. In men, the number of morphologically normal spermatozoa is usually reported to be below 10% and in animals above 50%. For example, breeding bulls and boars most often have <10% abnormal spermatozoa,52
and abnormalities are often more subtle than the severe abnormalities frequently seen in human samples. Even with relatively low numbers of normal spermatozoa, humans may still be able to reproduce. In contrast to wild animal species, where survival of the species may depend on a very high conception rate at each coitus, humans in monogamous relationships are not dependent on immediate reproductive success to the same degree. In fact, the current definition of couple infertility in most national health systems is ‘more than 1 year of regular, unprotected sexual relationship without pregnancy’.53
In other words, absence of pregnancy in spite of regular coitus during up to 12 ovulation periods can be considered ‘normal’ from a clinical point of view. However, fecundity may still be reduced compared to couples where conception occurs immediately after unprotected intercourse during the first cycle.
In conclusion, our large prospective study of men of the general population supports previous suggestions of a temporal decrease in semen quality, but it also indicated a recent small increase in sperm concentration and total sperm count. Follow-up studies are needed to detect if the upward trend is a real biological phenomenon or merely random variation. It is noteworthy that only one in four men had optimal semen quality from a fecundity perspective. Approximately 25% had a reduced quality compatible with prolonged waiting time to pregnancy, and another 15% had so severely impaired quality that they have a high risk of the need for fertility treatment to become biological fathers.