A healthy man makes about 80 million sperm a day, or 100 a second. But these numbers can change with age, and by season, and dwindle for any number of reasons – if a man gets too hot or has too little sleep, has a car accident or, of course, has sex – suddenly his count lands in the low range.
A good study should have men abstain from ejaculation for two to five days to collect an accurate sample – but it's difficult to know for sure. “The most striking thing missing from [nearly half the papers in the Danish report]was the duration of abstinence prior to the sample collected,” says Vancouver's David Mortimer, past president of the Canadian Fertility and Andrology Society.
In his controversial book The Skeptical Environmentalist, Bjorn Lomborg, a former member of Greenpeace, argues that semen samples collected in the 1930s before the sexual revolution and the birth-control pill should not be compared with those gathered after it: Modern men are far more sexually active, which might also explain why sperm counts seem lower.
Besides, sperm counts are “one of the least useful measures of fertility,” compared with how well a sperm swims and its shape, Dr. Mortimer says. The trouble is that picking individual sperm out of the crowd under a microscope is tricky.
“It's like looking down on a very busy street from the 11th floor on Bay Street – you see cars … but you don't know anything about their engines,” says Sergey Moskovtsev of the Create Fertility Centre in Toronto.
Labs are also notorious for miscounting sperm. Four times a year, the American Association of Bioanalysis sends out batches of the same semen sample to 500 U.S. labs for proficiency testing, Dr. Lamb says. Most fail.
Dr. Mortimer cites a study in which two labs tested a sample from the same man and found that “at the one lab the fellow would be identified as infertile, at another he could be a sperm donor.”
Still, he adds, “I would say there probably is a decline in sperm counts worldwide – but I would not stand up in a court of law and say so.”
Nor can anyone at the moment, says McGill's Dr. Robaire, who also serves as editor-in-chief of the Biology of Reproduction Journal. “I've seen too much garbage, too many bad studies.”
His own reading of the numbers is that counts clearly fell after the Second World War, stabilized and then picked up slightly. The big postwar dip coincides with women picking up smoking, he suspects, as animal studies show maternal smoking can reduce the number of stem cells in the developing male that make sperm. (Danish women had the highest smoking prevalence in Northern Europe after the war, studies find, peaking at about 40 per cent in pregnant women in the 1980s.)
In Canada, meanwhile there have been no national sperm-count studies at all. “No one's bothered to put it together,” Dr. Mortimer says. “The drive has not been there.”
Yet, as the experts like to say, testes are like those old canaries in a coal mine – they die first if there's trouble. “Semen is a reflection of overall health and well-being,” Dr. Moskovtsev says.
Down for the count
Over time, the World Health Organization has dramatically lowered the bar for a normal sperm count, based on sperm concentrations among men who impregnated a woman in a one-year period of unprotected intercourse. The guidelines used to peg the low end of fertility at 60 million sperm per millilitre, Dr. Moskovtsev says. Then it fell to 40, then 20 and in the most recent edition, published last year, it sits at 15 million.
Some researchers worry that this is a move in the wrong direction. In a letter published last week in the journal Science, Dr. Sharpe wrote: “It is time to stop accepting low sperm count as normal and confront the possibility that the fertility of present and future generations is at risk.”