Not every coitus leads to pregnancy and fertilisation.Give reasons
For Fertilisation to occur,the egg from ovary releases into the fallopian tube and reaches to the ampullary-ishthmic junction of the fallopian tube and wait for the sperm to come.The egg survives for only 24 hours.so,if a sperm come in contact with the egg in this duration then fertilisation will ocur and leads to pregnancy later.but this does not happen each time you can understand why.