In: Anatomy and Physiology
How are identical twins formed? What causes some identical twins to have different placental and amnion? What causes some identical twins to share amnion but not placental?
After fertilization of the egg with the sperm the embryo is formed. The formed embryo now sticks to the womb and then splits into two. Now we have two separate embryo of same genetic content and thus identical twins are formed.
According to when the zygote splits we can know that the twins share same or different placenta. If the splitting takes place within three days of fertilization the morula formed becomes blastocyst and it splits and now the both new blastocyst implants to the womb and thus the two feutus will grow in different placenta and the amniotic sacs will be different for those childs.
Identical twins with same amnion and different placenta is not possible but with same placenta and same aminon is possible. The morula formed with cell growth becomes blastocyst and get implanted in the womb. Then they split after one week of implantation and leads to the formation of same amnion. Like wise the twins with same placenta and different amnion is possible. I'm that case the blastocyst implants to the womb and splits as early as possible giving rise to same placenta and different amnion.