The process by which sperm and egg unite to form a zygote is called fertilization. Human fertilization occurs in the woman’s fallopian tubes. 1.5-4.0 ml of semen is secreted each time and contains about 40-100 million (4-10 crore) spermatozoa. Only one of these sperm unites with the egg. If fertilization does not occur within 12-24 hours of intercourse, the sperm die.