To join with an egg, sperm cells must complete 20-30 cm (8-12 in) journey from the woman’s vagina, via the uterus, to the egg in her fallopian tube. This is the equivalent of a 10-km (6-mile) swim for a human, millions of sperm begin the journey, but just a few survive to reach the egg – and only one will fertilize it.
Stage 1
The race begins
The vagina is an acidic environment for the sperm. Millions don’t make it beyond this stage.
Stage 2
Through the gap
The surviving sperm swim through the entrance to the uterus, which is slightly open.
Stage 3
The race gets rough
Many sperm fail to get through the cervix’s protective mucus. The successful ones now face attack from immune system cells in the uterus.
Stage 4
The last effort
Uterus muscles contract to push the sperm towards the fallopian tubes. Half of them swim towards the correct tube, where the egg is.
Stage 5
Fertilization
A handful of sperm arrive at the egg. Only one succeeds in burrowing through the egg’s outer layer to fertilize it.
Picture Credit : Google