The question of whether sperm concentration and quality is falling has been debated since the 1990s. Studies involving data going back to the 1950s show a drop in sperm concentration in industrialised countries, with exposure of the developing male foetus to low doses of endocrine disruptors - chemicals that interfere with hormones - sometimes suspected as behind the trend.
In the latest study, researchers collected data about 26,609 French men held by assisted reproduction centres across the country.
They examined semen samples provided by these men who were the partners of women undergoing fertility treatment because their Fallopian tubes were blocked or missing. In other words, the couples' infertility was due to the woman being infertile rather than to any problems with the men's sperm.
Falling quantity and quality
They found that over the 17 year period there was a significant and continuous 32.2% decrease in semen concentration, as measured by millions of spermatozoa per millilitre of semen. This broke down as an annual rate of decline of about 1.9% each year.
The researchers calculated that in men of the average age of 35, semen concentrations declined from an average of 73.6 million/ml in 1989 to 49.9 million/ml in 2005.
In addition, the percentage of normally shaped sperm fell by 33.4%. However, the researchers acknowledge that changes in the way sperm shape was measured during this time may partly explain this decrease and make it difficult to give an estimate for the general population.
The authors acknowledge that the 2005 sperm count still lies within acceptable fertility levels, as defined by the World Health Organisation (WHO). One of the authors, Joëlle Le Moal, an environmental health epidemiologist, says in a statement: "However, this is just an average, and there were men in the study who fell beneath the WHO values. The 2005 values are lower than the 55 million/ml threshold, below which sperm concentration is expected to influence the time it takes to conceive."
The authors took into account some factors that could affect the results, such as the men’s ages, the season, the centre where they gave their sperm samples, and the type of assisted fertility technique. However, they were unable to control for socioeconomic factors, including smoking and weight, which can affect semen quality and concentration.
Allan Pacey, senior lecturer in andrology at the University of Sheffield, says he is sceptical about the results. "I certainly don't believe it; I think it can mostly be explained by changes in laboratory technique," he tells BootsWebMD.
He adds: "I really don't think it's possible to look back in the archives almost 20 years and say with any confidence that what we were doing in the laboratory was remotely as good as what we are doing now; and the inevitable consequence of improving your technique is that you would see sperm counts go down, because poor technique tends to overestimate sperm concentration."
Allan Pacey says the jury is still out on the issue of whether sperm concentration and quality is declining.
The authors of the French study acknowledge that more research is needed.
'Decline in semen concentration and morphology in a sample of 26 609 men close to general population between 1989 and 2005 in France', Human Reproduction.
Press release, European Society of Human Reproduction and Embryology.
Allan Pacey BSc, PhD, Academic Unit of Reproductive and Developmental Medicine, University of Sheffield.
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