A sperm’s journey from vagina to egg is only 15cm long, but it’s a race with attrition.
There are approximately 200 million sperm in each human ejaculation, but only about two million sperm make it into the cervix. The rest are killed by the acidic fluids inside the vagina or lost in “flowback” — which is exactly what it sounds like.
Out of the approximately two million sperm entering the cervix, only about one million make it into the uterus. The rest are stopped by gooey mucus, or swim into dead-end channels inside the walls of the cervix.
Out of the approximately one million sperm that enter the uterus, only about 10,000 make it to the top of the organ. The rest are attacked and absorbed by white blood cells, which start to appear in force as soon as sperm enter their defensive perimeter.
Out of the approximately 10,000 sperm cells that make it to the far end of the uterus, only about 5000 turn in the right direction. The rest head towards the other oviduct, and unless both the woman’s ovaries have released eggs simultaneously (a rare event), those sperm are out of luck.
Out of the approximately 5000 sperm that enter the uterotubal junction — a twisty space connecting the uterus and the oviduct — only about 1000 enter the Fallopian tube. The rest get caught in the mucus lining the junction.
Out of the approximately 1000 sperm that enter the Fallopian tube, only about 200 reach the egg. The rest get attached to the lining of the oviduct, or just give out and die.
Out of the approximately 200 sperm that reach the egg, only one enters the egg to fertilise it. The rest are pushed away by the zona reaction, which makes the fertilised egg impermeable to additional sperm. Those last extra sperm? Yet more fodder for a woman’s immune system.