If all men in the world lost their ability to produce sperm, humans would surely be in danger of surviving. But previous studies have suggested that sperm quality could be declining. A new study, following up on a 20-year cohort, suggests that exposure to a number of factors in utero and during early life could lead to lower quality semen in adulthood, and a potential decline in male fertility.

The study was based on a follow-up of the Western Australia Pregnancy (Raine) Cohort, which began between 1989 and 1991, with the enrollment of 2,900 pregnant women. Part of the follow-up included testicular assessments for 423 of the men, aged 20 to 22. They were tested for testicular volume, semen quality, and hormone production. Their body composition was also tested for fat distribution.

The researchers found that one out of every six men had sperm quality that was below recent World Health Organization (WHO) standards for what it deems "normal." Out of all the men:

  • 14.8 percent were below the threshold of 1.5 milliliters seminal value
  • 18.9 percent had below the threshold of 39 million total sperm
  • 17.5 percent had below the threshold of 15 million sperms per milliliter of sperm
  • 14.4 percent had below the threshold of 32 percent motility - the ability for the sperm to move toward the egg.

What's more, 26.4 percent of the men had sperm that didn't meet WHO's criteria for appearance.

A number of factors lead to these below "standard" qualities. Those who were consistently small when in utero tended to be the ones who had worse quality sperm as adults, whereas better sperm quality was more likely in men who grew well in their mother's uterus. Men who were exposed to their mother's smoking as children — 18.6 percent of them — were also likely to have reduced sperm quality.

"The main message from our study is that to reach adulthood with the best possible testicular function a man should not be exposed to his mother's smoking, should have good fetal growth and, in childhood and through adolescence, should be 'appropriately grown' — that is, neither underweight nor overweight, and as an adult should not smoke or take drugs," Roger Hart, professor of reproductive medicine at the University of Western Australia, said in a press release.

Hart reported these results at the annual meeting of the European Society of Human Reproduction and Embryology.

Hart's message may be helpful for other reasons too. Men who are infertile also have the cancer risk of someone 10 years older than they are. Researchers followed 451 infertile men for about 6.7 years to see if they appeared in the Texas Cancer Registry.

The men had a condition called azoospermia, with about a quarter of them suffering from the non-obstructive type, in which sperm isn't produced by the testes. The rest of the men had obstructive azoospermia, which meant that their testes produced sperm, but it was blocked from being ejaculated.

They found that 29 men developed cancer an average of 5.8 years later. When compared to the general population, researchers would have only expected to see 16.7 cases.