It’s been almost 20 years since the researcher Niels Skakkebaek and his colleagues at the University of Copenhagen stunned scientists and the public alike by reporting that sperm counts were plummeting around the world. Skakkebaek had found that since the 1940s, the average sperm count had fallen from about 113 to 66 million per milliliter.
Although that analysis wasn’t without its critics, other scientists soon came up with similar findings (although recent evidence offers some good news: The precipitous decline in sperm counts may be leveling off). Researchers then began scrambling to identify a cause.
In today’s issue of the British newspaper The Independent, science editor Steve Connor summarizes what he calls an “emerging consensus” among some experts that the environmental causes behind the decrease in sperm production (and it must be environmental because the changes occurred too quickly to be genetic) may begin in the womb.
Or, as Connor put it: “It is not the lifestyle of men that is [the] problem, but that of their mothers.”
A number of studies point to a connection between early development in the womb and male reproductive problems in later life, especially low sperm counts. For example, men whose pregnant mothers were exposed to high levels of toxic dioxins as a result of the 1976 industrial accident in Seveso, Italy, have been found to have lower-than-average sperm counts. But men exposed to dioxins in adulthood showed no such effect. Another study found women who ate a large amount of beef during pregnancy, a diet rich in potentially damaging chemicals called polycyclic aromatic hydrocarbons (PAHs), had sons with relatively low sperm counts. But eating beef as an adult man shows no similar impact.
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 percent 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 percent — which also tends to be irreversible.
Of course, these studies cited by Connor are observational — they show only an association between two things (a mother’s meat-eating habits and her son’s low sperm count), not a cause-and-effect. Other, non-identified factors may have been at play.
Still, as Connor notes, there’s science to explain these findings:
Sertoli cells, which in the adult act as guardians for the development of sperm cells, are the very first cells to form from a “genital ridge” of the human male foetus. The number of sperm that can be produced in an adult man is critically dependent on the number of Sertoli cells that develop in his foetus, so anything that interferes with the formation of Sertoli cells in a mother’s will affect sperm production many years later.
Identifying the relevant lifestyle and environmental factors is the tricky part of this research. Leading candidates for study have been environmental chemicals that either mimic estrogen or block testosterone. But so far, reports Connor, only high exposure to these chemicals — like that in the Seveso study — have been found to have a clear link with low sperm counts.
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.