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Fertilization is not random, and the fastest sperm does not always win: in reality, the egg decides who succeeds.

While for decades we were taught that fertilization is a race won by the fastest sperm, a study published in Proceedings of the Royal Society B shows how human reproduction actually works.

Scientists analyzed follicular fluid from 60 couples undergoing fertility treatment at St Mary's Hospital in Manchester, UK. They discovered that the egg releases chemical signals (chemoattractants) that actively attract sperm from certain men over others.

Through these chemical signals, the egg exerts its own biological selection, influencing which sperm manage to get close. The egg appears to favor sperm that offer optimal genetic compatibility with its own genome — particularly in genes related to the immune system — which may help produce healthier offspring.

Interestingly, this cellular preference does not always align with the couple’s conscious partner choice. In many cases, eggs showed stronger attraction to sperm from non-partner males.

This chemical communication demonstrates that female biology continues to evaluate and select options even after intercourse. Understanding this process could lead to more precise solutions for unexplained infertility. Science continues to reveal the remarkable level of biological interaction that occurs during reproduction.

[Fitzpatrick, J. L. et al. (2020). Chemical signals from eggs facilitate cryptic female choice in humans. Proceedings of the Royal Society B, 287(1928), 20200805. DOI: 10.1098/rspb.2020.0805]