Get ready for parthenogenesis
By the middle of the XXI century, men may lose the ability to conceive children normally
Maria Azarova, Naked Science
A team of scientists from Israel, Denmark, Brazil, Spain and the USA conducted a meta-analysis that made it possible to update their previous data on a decrease in the concentration of sperm in men's sperm. It turned out that the average fell by 51.6% over 45 years. This is described in an article published in the journal Human Reproduction Update (Levine et al., Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries).
In 2017, the same publication published the work of the same group of researchers, in which they presented the results of an analysis of all publications in English for 1981-2013, containing information on the number of sperm in men in North America, Europe, Australia and New Zealand. The findings were alarming. However, then, due to a lack of data, scientists were unable to trace similar trends for residents of South and Central America, Asia and Africa.
In 2020, they decided to fill this gap and analyzed the results of several dozen more scientific papers, eventually receiving 223 studies that provided 288 assessments of sperm samples taken from 57,168 people from 53 countries on six continents in 1973-2018.
The participants were divided into two groups: the first included young men who did not know whether they were fertile; the second included those who had already become fathers and partners of pregnant women, including with the help of IVF, but regardless of the outcome of pregnancy. Men with confirmed infertility or low fertility (inability to conceive and/or give birth to a healthy child without medical assistance) were excluded from the sample.
According to the results of the meta—analysis, sperm concentrations decreased markedly - minus 0.87 million per milliliter annually, from 101.2 million per milliliter in 1973 to 49.0 million per milliliter in 2018. Among men from the first group, who lived on all the continents studied, this indicator dropped by 51.6% over 45 years. Moreover, the decline was especially noticeable in the data after the beginning of the XXI century: the process accelerated twice — from 1.16% after 1972 to 2.64% after 2000 annually.
Meta-regression models for the average sperm concentration (SC) and total sperm count (TSC) by year among men who did not know their fertility level from all continents, adjusted for potential distorting factors, for the entire period and limited to studies after 2000. A) The concentration of spermatozoa in semen. B) Total sperm count / © Human Reproduction Update
In general, the trends were similar for the total number of spermatozoa. Its sharp decline was revealed in the adjusted meta—regression model for all men - minus 2.06 million for the year.
"After stratification by fertility groups and adjustment for all covariates, including the continent, there was a strong decrease in the total number of spermatozoa among men who did not know their fertility level (minus 4.70), but not among fertile (minus 0.24). In men from the first group, the total number of spermatozoa from 1973 to 2018 fell by 1.40% per year and by 62.3% in general," the researchers said.
According to them, although both parameters studied — the total number of spermatozoa and their concentration in sperm — cannot be considered a reliable indicator of the ability to conceive a child, they are closely related. And the data obtained indicate a serious problem in public health.
"The relationship between sperm concentrations and time to conception is nonlinear. After the threshold of 40-50 million per milliliter, higher concentrations do not necessarily mean an increased likelihood of conception. On the other hand, below this threshold, the chances quickly fall. At the population level, the decrease in sperm concentration from 104 to 49 million per milliliter, which we are talking about in our work, implies a significant increase in the proportion of men with delayed conception time," the scientists explained.
Thus, if sperm concentrations continue to fall at the current rate, in the coming decades they will fall below the threshold of 40 million per milliliter. Then, according to Hagai Levin of the Hebrew University in Jerusalem, the first author of the new study, the crisis may reach a tipping point, and the consequences will become irreversible.
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