Sperm quality appears to be declining around the world but is a little discussed cause of infertility. Now scientists are narrowing in on what might be behind the problem.
“We can sort you out. No problem. We can help you,” the doctor told Jennifer Hannington. Then he turned to her husband, Ciaran, and said: “But there’s not much we can do for you.”
The couple, who live in Yorkshire, England, had been trying for a baby for two years. They knew it could be difficult for them to conceive as Jennifer has polycystic ovarian syndrome, a condition that can affect fertility. What they had not expected was that there were problems on Ciaran’s side, too. Tests revealed issues including a low sperm count and low motility (movement) of sperm. Worse, these issues were thought to be harder to treat than Jennifer’s – perhaps even impossible.
A hidden fertility crisis?
The global population has risen dramatically over the past century. Just 70 years ago – within a human lifetime – there were only 2.5 billion people on Earth. In 2022, the global population hit eight billion. However, the rate of population growth has slowed, mainly due to social and economic factors.
Birth rates worldwide are hitting record low levels. Over 50% of the world’s population live in countries with a fertility rate below two children per woman – resulting in populations that without migration will gradually contract. The reasons for this decline in birth rates include positive developments, such as women’s greater financial independence and control over their reproductive health. On the other hand, in countries with low fertility rates, many couples would like to have more children than they do, research shows, but they may hold off due to social and economic reasons, such as a lack of support for families.
At the same time, there may also be a decline in a different kind of fertility, known as fecundity – meaning, a person’s physical ability to produce offspring. In particular, research suggests that the whole spectrum of reproductive problems in men is increasing, including declining sperm counts, decreasing testosterone levels, and increasing rates of erectile dysfunction and testicular cancer.
Swimming cells
“Sperm are exquisite cells,” says Sarah Martins Da Silva, a clinical reader in reproductive medicine at the University of Dundee and a practicing gynaecologist. “They are tiny, they swim, they can survive outside the body. No other cells can do that. They are extraordinarily specialised.”
Seemingly small changes can have a powerful effect on these highly specialised cells, and especially, their ability to fertilise an egg. The crucial aspects for fertility are their ability to move efficiently (motility), their shape and size (morphology), and how many there are in a given quantity of semen (known as sperm count). They are the aspects that are examined when a man goes for a fertility check.
“In general, when you get below 40 million sperm per millilitre of semen, you start to see fertility problems,” says Hagai Levine, professor of epidemiology at the Hebrew University of Jerusalem.
Sperm count, explains Levine, is closely linked to fertility chances. While a higher sperm count does not necessarily mean a higher probability of conception, below the 40 million/ml threshold the probability of conception drops off rapidly.
In 2022, Levine and his collaborators published a review of global trends in sperm count. It showed that sperm counts fell on average by 1.2% per year between 1973 to 2018, from 104 to 49 million/ml. From the year 2000, this rate of decline accelerated to more than 2.6% per year.
Levine argues this acceleration could be down to epigenetic changes, meaning, alterations to the way genes work, caused by environmental or lifestyle factors. A separate review also suggests epigenetics may play a part in changes in sperm, and male infertility.
“There are signs that it could be cumulative across generations,” he says.
The idea that epigenetic changes can be inherited across generations has not been without controversy, but there is evidence suggesting it may be possible.
“This [declining sperm count] is a marker of poor health of men, maybe even of mankind,” says Levine. “We are facing a public health crisis – and we don’t know if it’s reversible.”
Research suggests that male infertility may predict future health problems, though the exact link is not fully understood. One possibility is that certain lifestyle factors could contribute to both infertility, and other health problems.
“While the experience of wanting a child and not being able to get pregnant is extraordinarily devastating, this is a much bigger problem,” says Da Silva.
Individual lifestyle changes may not be enough to halt the decline in sperm quality. Mounting evidence suggests there is a wider, environmental threat: toxic pollutants.