Twin to Twin Transfusion
Twin to Twin Transfusion Syndrome is a
very serious complication affecting approximately 15% of
identical twin pregnancies.
Twin to twin transfusion
syndrome is a complication of the placenta which can only happen
in an identical multiple birth pregnancy (ie: twins that share
the placenta) The blood flows through connections in the
placenta, pumping the blood from one twin (donor) to the other
twin the (recipient). Making the donor twin at risk of heart
failure from anaemia and the recipient twin at risk of heart
failure from the excess blood.
A twin is one of two children produced in the same pregnancy. Twins can either be identical (in scientific usage, "monozygotic"), meaning that they develop from one zygote that splits and forms two embryos, or fraternal ("dizygotic") because they develop from two separate eggs that are fertilized by two part sperm.
In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple. It is in theory, possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother and father were to be exact matches from one birth to the next. While this is statistically unbelievable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may sometime be possible.
Twin-to-twin transfusion syndrome is a difficulty of disproportionate blood supply, resulting in high morbidity and mortality. It can affect monochorionic multiples, that is multiple pregnancies where two or more fetuses share a chorine and hence a single placenta. Severe TTTS has a 60 to 100% mortality rate.
Incidence TTTS occurs in approximately 5.5 to 17.5% of all monochorionic pregnancies. Based on 2003 CDC data, the theoretical incidence of TTTS would be 1.38 to 1.86 cases in every 1,000 live births or an estimated near 7,500 cases each year in the United States. One Australian study, however, noted an occurrence of only 1 in 4,170 pregnancies or 1 in 58 twin gestations. This distinction could be party explained by the "hidden mortality" associated with MC multifocal pregnancies--instances lost due to premature rupture of membrane (PROM) or intrauterine fetal demise before a thorough diagnosis of TTTS can be made.