Although the way sperm shape is measured changed during the course of the study, the researchers pointed out these changes do not explain the entire 33 percent decline in normally formed sperm. They added that sperm motility or movement increased slightly -- from 49.5 percent to 53.6 percent over the course of the study.
Although the researchers considered the men's age, the time of year and the fertility center the men used, they noted that they were not able to take other factors into account, such as weight and smoking, which can affect sperm concentration and quality. They pointed out that assisted reproduction technology is typically used by those with more education who are also less likely to smoke or be overweight.
"Therefore, the real values for sperm parameters in the general population could be slightly lower than those that we present and the decreases could possibly be stronger," the study's authors wrote.
The researchers said their findings support studies from other countries that revealed similar results. They concluded that more research is needed to explore the possible causes for the decrease in semen concentration and quality. Previous studies suggested that environmental factors, such as a disruption in the body's hormonal balance, may play a role in men's fertility and could affect future generations.
"Our public health warning may help health authorities to reinforce their actions on endocrine disruptors [certain environmental chemicals], hopefully at the European level, and to sustain research as well as monitoring systems," said Le Moal. "Our example could help other countries to implement their own systems. International monitoring systems could be a good idea to understand what is happening on human reproductive outcomes a
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