Why levels of sperm in men are falling

Article here. Excerpt:

'As many as one in five healthy young men between the ages of 18 and 25 produce abnormal sperm counts. Even the sperm they do produce is often of poor quality. In fact only between 5 and 15 per cent of their sperm is, on average, good enough to be classed as "normal" under strict World Health Organisation rules – and these are young, healthy men. By contrast, more than 90 per cent of the sperm of a domestic bull or ram, or even laboratory rat, are normal.

Human males also suffer a disproportionately high incidence of reproductive problems, from congenital defects and undescended testes to cancer and impotency. As these also affect fertility, it's a minor miracle men are able to sire any children at all. In fact, an increasing number of men are finding themselves childless. Among the one in seven couples now classed as infertile, the "male factor" has been found to be the most commonly identified cause.
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There is now an emerging consensus among some experts that whatever it is that is exacerbating the problems of male infertility, it probably starts in the womb. It is not the lifestyle of men that is problem, but that of their mothers.
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Meanwhile, studies of migrants between Sweden and Finland, showed that a man's lifetime risk of testicular cancer tends to follow the country he was born in rather than the country where he was brought up. It was his mother's environment when she was pregnant with him, rather than his own as a boy or as an adolescent, that seems to have largely determined a man's risk of testicular cancer.

One of the strongest pieces of evidence in support of this idea comes from studies of people who smoke. A man who smokes typically reduces his sperm count by a modest 15 per cent or so, which is probably reversible if he quits. However, a man whose mother smoked during pregnancy has a fairly dramatic decrease in sperm counts of up to 40 per cent – which also tends to be irreversible.
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"Public concern about the adverse effects of environmental chemicals on spermatogenesis in adult men are, in general, not supported by the available data for humans. Where adverse effects of environmental chemicals have been shown, they are usually in an occupational setting rather than applying to the general population," he says.

So although scientists are closing in on the critical window of foetal development in the womb that determines a man's fertility status in later life, they are still not sure about what it is that could be affecting this change in his reproductive status. But one thing is clear, it is his mother who almost certainly holds the key.'