Today, IVF clinics try to select the best sperm for in vitro fertilization with a microscope by looking at how they swim. They are then centrifuged through liquids of different thicknesses to find the strongest Sims. However, the test environment does not resemble the environment in the uterus and fallopian tube.

"Sperm has many different challenges along the way. It must swim in liquids that are thin or thick. It has to navigate through the complex maze of a woman's genitals and then it has to penetrate the egg for it to be fertilized, says fertility researcher Meurig Gallagher of the University of Birmingham.

Sperm swimming style reveals the best candidate

In a new study, sperm were allowed to swim in a channel where current velocity and fluid thickness could be changed to see which swims best in different environments. In the video, the researchers look at them using a dark-field microscope that illuminates the sperm so you can see their swimming style clearly.

Whether this technique can be used in IVF clinics needs to be evaluated to see if it leads to more children.

"I think it can be an interesting tool for studying fundamental scientific questions about what distinguishes a good sperm. We're not looking for sperm that can swim very quickly in one direction and do nothing else," says Meurig Gallagher, who was not involved in the current study.