Abnormalities Unique to Multiple Gestations
· Intrauterine death of one twin- Usually occurs in the first trimester, although can occur at a later stage. The loss rate of one twin is about 20%. Diagnosis is frequently sonographic —the ‘vanishing twin’. If the fetus is lost in the second trimester it may become mummified –fetus papyraceus. Loss in later pregnancy results in significant fetal loss or handicap in the surviving twin if monozygotic.
· Twin-twin transfusion syndrome- This occurs in MC twins due to arteriovenous anastomosis in the shared placenta. The donor twin is small and anemic. The recipient is polycythemic and large, being at risk of high output cardiac failure.The mortality rate is 40—90% with both twins at risk. US findings include: (1) discordance in fetal weight 25%; (2) discordance in amniotic fluid volume; (3) fetal hydrops in 10—25% of recipient twin; (4) disparity in the relative size and number of cord vessels; (5) single placenta with areas of disparity in the echogenicity of the cotyledons supplying the two cords.
· Acardiac twining- Occurs in 1% MC twins; one fetus develops without a functioning heart and other severe deformities are associated. The most common variety is ahs; head and heart — acardius acephalus. US reveals a twin pregnancy; one fetus has an absent or rudimentary heart; other anomalies may be present. Diffuse skin thcheing is usually seen.
In the acardiac twin the umbilical cord often has one artery and one vein. Reversed direction of flow can be shown by Doppler US. US helps in management by looking for evidence of heart failure in the normal twin. Early delivery can arranged if there is fetal compromise, if lung maturity is evident.
· Abnormalities in blood flow-
· Twin embolization syndrome- The death of an MC twin may be associated with thrombotic or embolic episodes in the live twin. The thromboembolic material originates from the dead fetus and passes into the live fetus via intraplacental vascular connections. This results in neurologic deficit, and vascular damage to the kidneys and small bowel.