Fertilisation is the process of union of the sperm and the ovum inside the female reproductive tract. The site of fertilisation in humans is the junction between ampulla and isthmus of the fallopian tube. Thus, for fertility, it is necessary that the ovum be present at this site, when the sperm reaches and has not crossed it. As the release of ovum from the ovaries takes place between 11^th to 16^th day of menstrual cycle, which is roughly the middle of the cycle, the chances of ovum being present in fallopian tube at that time is high. Hence, fertilisation is possible when ovulation occurs, in the middle of menstrual cycle.