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Name: Amy B. C.
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: December 2002

Is it possible for Siamese twins to be fraternal? I thought fraternal twins had their own amniotic sacs, so the fusing of the two would not happen. Identical twins can grow in their own, or individual sacs (per my cousin, who is an internal medicine doctor). If they are Siamese, they would obviously be in the same sac. I do not like to give out false information, however, so I wanted to check with an expert. What background do you have to answer this question?

The medical term for "Siamese twins" is conjoined twins. They can only form if they are identical. This is because of its cause. When a sperm fertilizes an egg it becomes a new cell called a zygote. Shortly thereafter, the zygote begins to divide from one cell to two, two to four, etc. This takes place in a jellylike coating called the zona pellucida. Occasionally, the zona pellucida also divides separating the cell masses, which then begin to divide also. Because these two cell masses were from the first original cell, the zygote, they are genetically identical and are called monozygotic twins. Not all monozygotic twins share the placenta and/or amniotic sac. It depends on when in development the split occurred, so they can still be monozygotic and have totally separate placentas and amniotic sacs although this is rare.

Usually human females only ovulate once per month. Occasionally, more than one egg (ovum) is ovulated. Each of these may be penetrated by separate sperm and are no more alike than other siblings. They just happen to be born on the same day (usually)! They can even be from different fathers. These are called dizygotic twins. Very rarely, the cell mass divides after the point at which the cells have committed to becoming certain tissues and the cells are not "totipotent" anymore. This means that the parts must be divided between the cell masses and they must share certain structures. How many organs they share can help the doctors determine when this division occurred. So, no it is not possible for conjoined twins to be fraternal.



My understanding of this comes as an interested reader on the subject, I do not have any special experience or training in medicine.

Conjoined twins (formerly called siamese) occur due to the incomplete division/development of one embryo/fetus apart from the other, with both originating from the same fertilized ovum. Because of this, if a set of twins is found to be conjoined, they would have to be identical twins; there would be no way for fraternal twins to fuse and share organs as is found in the conjoined form.

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