Question: What is that in humanbody that makes either fraternal and identical twins have some of their body parts fused to each other

Cheryl Andisi answered on 25 Jan 2017:

@aaab279,
Conjoined twins is a very rare birth defect that occurs while the embryo is developing. Conjoined twin share the same amniotic sac and placenta during development in the uterus. It is thought that the joining of body parts is as a result of either problems during fission ie separation of cells or fusion, where similar cells join during embryonic development.
After fertilization, the one cell zygote formed undergoes many rapid mitotic cell divisions-called cleavage, before the distinction of the different cell layers occurs and differentiation- development of different organs starts. During cleavage, daughter cells are identical to their parent cells but all remain “stuck together”. If these cells happens to separate, then each will continue carrying out its cell divisions and go on to produce an embryo and eventually twin babies. If this separation is not complete, it will result in conjoined twins.
Since these twins are formed from the same egg and sperm, they are usually the same genetically and will be of the same sex. Most conjoined twins arrive as still births. In addition, even though there are more male conjoined twins in the uterus, the number of female conjoined twins after birth is 3 times higher which might imply that they have better survival rate.
There are different types of conjoined twins, depending on where they are joined. This also determines the success of possible surgical separation.