Fertilization normally occurs in the ampulla of the uterine tube within 24 hours of ovulation. It is a complex interaction between male and female gametes that forms a zygote. The zygote then undergoes cleavage as it travels through the fallopian tube to the uterus. Around 6-7 days after fertilization, the embryo implants itself into the uterine wall, forming the placenta and embryo from the trophoblast and inner cell mass respectively.