Infertility specialists estimate that about 10% of couples trying to conceive have fertility issues.
Many factors can influence the ability to get pregnant.
Now, a new study from an evolutionary biologist at Tel Aviv University's Department of Zoology suggests that evolution is causing the reproductive organs of men and women to become reproductive adversaries, causing higher than expected infertility rates.
Dr.
Oren Hasson believes that evolution should have improved humanity's reproductive success rate by now, but that has not occurred.
Working with Prof.
Lewi Stone of the department's Biomathematics Unit, the two combined empirical evidence with a mathematical model developed by Prof.
Stone.
Biological Reviews journal recently published the researcher's conclusions.
Throughout thousands of years of evolution, women's bodies have rewarded more aggressive sperm with penetration of the egg.
In response, men's bodies have developed super-sperm, faster, stronger swimmers.
Thus, men over-produce these competitive sperm to increase the possibility of successful fertilization.
However, there are unintended consequences, it's a delicate balance, and over time our bodies fine tune to each other.
Sometimes, during the fine-tuning process, high rates of infertility can be seen.
That's probably the reason for the very high rates of unexplained infertility in the last decade, according to Dr.
Hasson.
Timing may be the problem.
Normally, one sperm penetrates and binds with the egg, activating biochemical changes that block the entry of other sperm.
This occurs because more than one penetrating sperm will kill the egg.
However, with today's over-aggressive sperm, polyspermy may be occurring, more than one sperm may be entering the egg before blocking is completed, terminating the fertilization process.
Polyspermy can be an issue in IVF (in vitro fertilization) therapy.
Fertility doctors sometimes perform ICSI (Intracytoplasmic Sperm Injection), injecting a single sperm into the female ova when treating male factor infertility, avoiding polyspermy in the lab.
Dr.
Hasson explains that women's bodies have been developing defenses against this condition.
To avoid the fatal consequences of polyspermy, female reproductive tracts have evolved to become formidable barriers to sperm, and they eject, dilute, divert and kill spermatozoa so that only about a single spermatozoon gets into the vicinity of a viable egg at the right time.
This evolutionary cycle continues, Dr.
Hasson argues, with efficiencies in male sperm being matched by responses in the female reproductive system.
He also points out that sperm have become more sensitive to stress and pollution, which nature could not have anticipated, since natural selection is short-sighted.
He believes that IVF specialists can increase the odds of conception by more carefully computing the number of sperm placed near the female ova.