Prezygotic refers to any stage of the reproductive process before the formation of the zygote, while postzygotic refers to any point of life after the ovum has been fertilized. Both terms are used when describing the fertilization and conception processes.
A zygote is the genetically unique fusion of female and male reproductive cells. It contains all the genetic information required to develop into a baby. The zygote travels down the Fallopian tube until it attaches to the uterine wall; at this point, it is considered a living organism since it goes through the four criteria of life: it reacts to stimuli, exhibits growth, reproduces and has metabolism. The concept of postzygotes and prezygotes is widely used in the theory of reproductive isolation.