A new study has shed some light on what happens when a female egg meets the sperm - and the results are fascinating.
If the egg likes the sperm, it sends chemical signals that tell it to swim faster. However, if the egg doesn't like the sperm, the signals it sends encourage the sperm to slow down.
It's proper law of attraction at work, with the egg actually ghosting the sperm it's not interested in.
The study refutes the long-held belief that the strongest, fastest sperm gets the egg.
Researchers from Stockholm University and the Manchester University NHS Foundation Trust found that the egg effectively chooses the sperm it wants, and rejects the others.
"Human eggs release chemicals called chemoattractants that attract sperm to unfertilised eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract," said John Fitzpatrick, an Associate Professor at Stockholm University.
Once the sperm makes it up to the fallopian tube and meets the egg, it comes into contact with the chemoattractants in the follicular fluid.
The researchers found it is likely the strongest sperm survives but merely because the egg chose to have that happen.
"When sperm go into the follicular fluid, they start to go straighter and they start to change the way they swim. So, depending on the strength of that signal, you can get different responses in how the sperm are responding to these female chemical signals within their follicular field," Professor Fitzgerald said.
The study also looked at how the sperm responds to follicular fluid, the fluid that surrounds the eggs. It found that fluids from different females attract sperm from some males more than others.
"The idea that eggs are choosing sperm is really novel in human fertility," said Professor Daniel Brison, the scientific director of the Department of Reproductive Medicine at Saint Mary's Hospital and a senior author on the study.
"Research on the way eggs and sperm interact will advance fertility treatments and may eventually help us understand some of the currently 'unexplained' causes of infertility in couples."