Human twins may be similar, when they are formed from one egg (monozygotic) or fraternal while they are the result of fertilization of two different ova released at the same time and fertilized through different sperms (dizygotic): From an embryological point of view identical twins are of greater interest. 0.25% of all births might be monozygotic twins. Current evidence indicates that twinning takes place at the blastocyst stage by the splitting of the inner cells mass. The cells of the ICM are refer totipotent i.e. they have the potential of forming a single dual or even a multiple embryonic disc. If by chance, the separation of inner cell mass takes place before day 5, even before the formation of trophoblast, which occurs in 33 percent of similar twins, then they have two separate chorion and amniotic cavities. (Figure (a)).
Identical twins which share a common chorion, but two separate amniotic cavities indicate that the split in ICM came among the formation of chorion on day 5 and formation of amnion on day 9 as displayed in Fig.(b). This is the case in two thirds of identical twins. A small percentage of identical twins are born in a single chorion sharing single amniotic cavity as shown in Fig (c). This points out that the division occurred after day 9 after the implantation of the blastocyst. As seen in Fig (d), the embryonic disc may divide not completely i.e., the two new embryonic axes fail to separate completely and the outcome is conjoined twins. The degree of union may be little or quite extensive and the twins may be joined at any part of their body. It should be kept in mind, however, that conjoined twins represent an aberration in development that would usually lead to formation of identical twins. Most conjoined twins do not survive after birth but nowadays it is possible to separate some of them surgically and then one or both may survive to lead fairly normal lives.