No one knows why an embryo splits to make identical twins, so we don't know why TTTS happens. What we do know, is that the later, in days after conception, that the embryo splits, the more problems that are seen including the syndrome. The occurrence of embryo splitting is out of our control. Most twin to twin transfusion syndrome pregnancies are seen with monochorionic-diamniotic twins. It is difficult to answer this question with certainty, because formal tracking mechanisms to record TTTS pregnancies are only now being established.
Many pregnancies go undiagnosed or unrecorded.
It is crucial to have the placenta analayzed to confirm TTTS and to aid in realistic numbers. This in turn helps to further medical research and helps us prove the seriousness and urgency in fighting this condition. It is important for parents to understand that the babies are normal. They only become affected by what is happening in the placenta. Just as the mother has no control in when the embryo splits and how the placent forms, the babies have no control on how they each receive their nourishment.