Sperm counts are falling, the population is allegedly in peril. How long will men remain fertile? The question has been asked seriously in medical journals since the early 1990s, with a decrease both in sperm quantity and quality being observed. But what do these readings mean?

The favoured method of measuring male fertility is through the microscopic analysis of semen. Observing a fresh sample under the lens takes account of volume, concentration and movement. Any amount over 20m sperm per millilitre is normal, and more than 85 per cent of sperm can be of abnormal shape before the sample is declared possibly suboptimal. (Note that what is “normal” or not has to take account of the effect of error in handling and counting large numbers.)

In any case, why do some people believe sperm counts are falling? From environmental toxins to radiation from mobile phones, to obesity and oestrogen-disruptive chemicals, a variety of explanations are offered. But while some studies have backed up these hypotheses, others – from places as diverse as Japan, Copenhagen and Seattle – show no significant changes over the past couple of decades.

Perhaps we have to ask what constitutes a normal count. A paper in the journal Urology in 1996 outlined world variations in sperm counts – and they are huge: from an average of 52.9m sperm per cc in Thailand to 120.9m per cc in France. Given evidence such as this, the significance of the fall in, say, British males’ sperm count becomes less clear.

The truth is that laboratory values for “subfertile” and “fertile” samples overlap to a large extent. A study in the New England Journal of Medicine in 2001 explained that “threshold values for sperm concentration, motility and morphology can be used to classify men as subfertile, of indeterminate fertility, or fertile. None of the measures, however, are diagnostic of infertility.”

The test is whether the man can conceive. There are men with low or borderline sperm counts who have little difficulty in creating a pregnancy. And then there are men with “normal” counts who find conception elusive.

We don’t just need to know if sperm counts are falling or not, but if the number of conceptions they produce are being affected. The Lancet examined this question in 2000: how long did it take representative ages of couples to become pregnant between 1961 and 1993? They found a general rise in fertility: apart from a couple of dips, the trend was for couples that tried for pregnancy achieving it earlier. This is good news. Clearly, we need more quality research; but we also need to know the real-life implications of it.

Margaret McCartney is a GP in Glasgow
margaret.mccartney@ft.com

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