Fraternal Siamese Twins?
Name: Amy B. C.
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
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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