The fertilization of a woman’s egg occurs when a sperm cell penetrates through the wall of the egg cell. We often hear about this process as a race between the sperm cells to travel up through the uterus and the fallopian tube to reach the unfertilized egg. However, new research found that it may not be entirely random which sperm is allowed to fertilize the egg.

It seems that the egg cell is active in the selection process and purposefully chooses to be fertilized by certain sperm cells – which are not necessarily those that come from the woman’s partner. So far, the conviction has been that the egg does not choose a partner itself.

The research study has examined how different men’s sperm cells responded to the follicular fluid from 16 different women. Follicular fluid is the fluid released with the egg during ovulation and which surrounds the egg in its journey from the ovary to the uterus. This liquid contains signaling molecules that the egg has released.

These findings suggest that one woman’s follicular fluid was better at attracting semen from some men, while another woman’s follicular fluid was better at attracting semen from other men. Which sperm the egg ‘preferred’ did not depend on who the woman’s partner was. The egg does not necessarily agree with the woman as to who is to fertilize it. This interesting study’s findings may be part of the reason why some couples are inexplicably childless. 

Source: https://royalsocietypublishing.org/doi/10.1098/rspb.2020.0805
https://www.dr.dk/nyheder/viden/kroppen/kvinders-aeg-favoriserer-bestemte-maends-saed-og-det-er-ikke-altid-partnerens