why don't all eggs and sperms get fertilised?
During a sexual intercourse, about 200-300 million sperms are ejaculated. For a normal fertility, atleast 60% of the sperms should possess normal shape and size and atleast 40% must be vigorously motile. Ovum consists of many layers, one of which is zona pellucida layer. During fertilization, when sperm (one with the most appropriate size, shape and motility) comes in contact with this layer, some changes are induced in the membrane that block the entry of additional sperms. Due to these changes, it is ensured that only one sperm can fertilise an ovum.