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Three random events occur in identical (monozygous) twins
that determine whether the twins develop
TTTS: the timing of the twinning process which
determines placental type, the number, type and
direction of the connecting vessels, and the degree
to which the twins share their common placenta
equally or asymmetrically. Most identical twins
have a monochorionic placenta and approximately
15% develop TTTS. The placenta findings determine when in
pregnancy TTTS occurs, the degree and
severity of the transfusion, and the outcomes (survival
with and without abnormalities in thebabies) with various
treatments. In addition to TTTS, the placental abnormalities
can cause theearly loss of one twin, as well as developmental
birth defects in one twin. Couples with monochorionic twins
are at high risk and should be examined often by their physician,
have frequent ultrasound scans (The TTTS Foundation advocates
weekly ultrasounds from
16 weeks through delivery), and plan to have careful placental
examination after delivery.
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