Twin-to-twin transfusion syndrome (TTTS, also known as Feto-Fetal Transfusion Syndrome (FFTS) and Twin Oligohydramnios Polyhydramnios Sequence (TOPS)) is a complication of disproportionate blood supply, resulting in high morbidity and mortality This condition occurs only in those identical twins that are monochorionic, diamniotic. In almost all of these pregnancies,. For reasons that are not clear, in 15-20% of monochorionic, diamniotic twins the single placenta contains blood vessel connections between the twins, the blood flow through these blood vessel connections becomes unbalanced resulting in a condition known as twin-twin transfusion syndrome (TTTS). Most pregnancies result in one baby. In about
1 in 80 pregnancies, twins are conceived. This can occur in one of two ways. The more common way is for the two different sperm to fertilize two different eggs resulting in what is called a dizygotic (DZ) twin gestation. These twins are often called fraternal twins. In this type of twinning each twin has its own sac of amniotic fluid and its own placenta (afterbirth). Dizygotic twins have two sets of membranes surrounding their amniotic fluid sacs and therefore they are known as diamniotic, dichorionic. In about 1/3 of twin pregnancies, one sperm fertilizes one egg but this splits into two
embryos resulting in what is known as monozygotic (MZ) twins. These twins are often referred to as identical twins since they have the same genetic material. Approximately 1/3 of MZ twins look just like fraternal twins on prenatal ultrasound since there are two separate amniotic sacs and two separate placentas. However in 2/ 3s of identical twins, each twin has its own amniotic sac but the twins share a common placenta. This type of MZ twinning is called monochorionic, diamniotic twins.
Key words: Twin-to-twin transfusion syndrome, Feto-Fetal Transfusion, Twin Oligohydramnios Polyhydramnios Sequence