The state of male fertility has been one of the most hotly debated subjects in medical science in recent years. While a number of previous studies found that sperm counts and quality have been falling, some dismissed or criticized the studies over factors such as the age of the men included, the size of the study, bias in counting systems or other aspects of the methodologies.

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Some of the other concerns are outlined in an analysis published by the American Society of Andrology, which focuses on the male reproductive system. The skepticism also has to do with the difficulty of comparing records from a fertility center in the 1970s with one from today and with the fact a single man's sperm count may fluctuate during his life span due to his weight, use of alcohol and many other factors.

However, Shanna H. Swan, one of the authors of the new study published in the Human Reproduction Update, said that the new meta-analysis is so broad and comprehensive, involving all the relevant research published in English, that she hoped it would put some of the uncertainty to rest. Then the scientific community could move forward into putting its resources into figuring out the why of what is going on, she said.

“It shows the decline is strong and that the decline is continuing,” Swan said in an interview.

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The analysis found drops only for men in Europe, North America and Australia and not for those in South America, Asia and Africa. Swan explained that this could mean that there's something specific to certain cultures or regions that affects sperm, but that it's also possible that there just isn't enough data yet to draw firm conclusions about the rest of the world. There have been far fewer sperm studies conducted in non-Western countries. (Some of the most recent ones have also noted a decline in aspects of sperm quality. A study published earlier this year about China's Hunan province found that 56 percent of sperm donations met the criteria for health in 2001 vs. only 18 percent in 2015.)

The most important data points in the new study involved sperm concentrations for what are known as “unselected” men who haven't yet proven they are fertile. These are men in the studies who are on the younger side and are not yet fathers or do not have partners who are pregnant. Researchers estimated that these men had an average sperm concentration of 99 million per milliliter in 1973 but that that had dropped to an average 47 million per milliliter in 2011.

That is a disturbing number given that, according to World Health Organization criteria, men with a sperm concentration of less than 40 million are considered to have an impaired chance of conceiving and those with a sperm concentration of less than 15 million per milliliter are unlikely to be able to have children.

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These numbers mean “surprisingly higher proportions of men are falling into the infertile and sub fertile categories,” Swan said.

There are numerous theories about what may be happening to sperm. Many scientists say the most sensitive period may be during the first trimester, when the developing fetus's reproductive system may be impacted by a mother smoking, stress she experienced or food she ate. Exposure to chemicals that can change hormone levels, known as endocrine disrupters, are among the issues being studied.

Over the life span, men are also exposed to a number of other things that could potentially influence sperm concentration: pesticides, lead, X-rays, stress and countless other factors.

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The changes in the womb can cause permanent damage, Swan explained, while the adult exposures are mostly reversible.

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The issue of sperm isn’t just about having babies.

It has larger implications for men's health. Poor sperm health has been linked to cardiovascular issues, obesity, cancer and more generally to higher rates of hospitalization and death. While men’s life expectancy is increasing overall thanks to advances in medical care, nutrition and sanitation, it isn’t unthinkable that that could one day reverse.

“Having a low sperm count is a signal,” Swan said, “that there’s something wrong in men’s health overall.”