(CNN) The race to fertilize an egg is a ruthless one -- and now, scientists have shed more light on the punishing obstacle course attempted by hundreds of millions of sperm cells in their bid to get there first.

Women's reproductive tracts are shaped in a way that rejects weaker sperm, allowing only the fastest candidates through a series of "gates," researchers at Cornell University have found.

The team, writing in the journal Science Advances , used a number of models and computer simulations to observe sperm along their journey from the cervix to the egg.

They noticed weaker sperm got caught in currents at the gates -- or "strictures" -- while the fastest and strongest swimmers were able to withstand the oncoming flow and force their way through the opening.

"Strictures inside the sperm swimming channel play a gate-like role," the authors explained. "Sperm with velocities higher than a threshold value can pass through the stricture, whereas sperm slower than the threshold accumulate below the stricture."

Read More