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The Twin to Twin Transfusion Syndrome Foundation
Chart 1 - Monozygotic Twinning
In human pregnancy if the fertilized egg (embryo) splits after four days, a particular type of identical twin pregnancy occurs known as monochorionic.
The later an embryo splits, the thinner the membrane and the more problems that may occur.
Most TTTS pregnancies are monochorionic-diamniotic.
They can also be monochorionic-monoamniotic. They are always monochorionic.
This chart shows the different ways an embryo may split to make identical, like sexed babies, depending on how many days after conception.
After 12 days, there would be siamese, or conjoined twins.
In these cases the twins share a unified placenta.
Studies of these placentas indicate that, almost invariably, communicating blood vessels are found within the placenta which connect the umbilical cords and circulation's of the twins.
it is approximately 15% of the time, that the sharing becomes so disproportionate or uneven that the syndrome occurs.
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