Zona pellucida constitutes the outer most layer of morula (16-celled stage embryo). During the formation of blastocyst, zona pellucida becomes thinner and finally disappears. Blastocyst consists of an outer envelop of cells called trophoblast and the inner cell mass. The trophoblast has a strong tendency to stick with the endothelium to which it comes in contact with. Zona pellucida prevents the sticking of the trophoblast with the endothelium during the journey of fertilised ovum from ampullary region of the Fallopian tube to the uterine cavity. Thus, the most important function of zona pellucida is to prevent implantation of the blastocyst at sites other than the normal, thus preventing ectopic pregnancy. As soon as the blastocyst becomes ready for implantation, zona pellucida disappears.