Introduction

Twin-to-twin transfusion syndrome (TTTS) may complicate multiple pregnancies where there is vascular conjoining within the hemochorial placenta, most commonly in monochorionic twins. The disease has fascinated obstetric specialists since it was first described in 1875 [1] when it was recognized as a condition not amenable to treatment and with very high perinatal loss rates. Since that time, this condition has offered many academic challenges. This is not only in terms of understanding the underlying pathophysiology but also a clinical challenge, in attempting to alter the clinical course of a condition that affects 10–15% of monochorionic twins. This is with a “backdrop” of a condition whose natural outcome would be at least 90% perinatal loss rates and very high (>50%) neuromorbidity in any surviving babies [2, 3].

TTTS is a relatively unusual condition as genetically concordant (i.e., monozygous) twins develop phenotypically discordant features as a consequence of monochorionic placentation and the conjoined angioarchitecture (see Chapter 11.2). In monozygous twins, placentation is dependent on the timing of the zygote splitting, post-fertilization of a single ovum and sperm. If this occurs after 72 hours, then a single placental disk develops, most commonly with two amnions. The “splitting” of the single cellular mass forms the most well-developed hypothesis in the formation of monozygous twins but a “co-dominant axis theory” (with the development of more than a single organizing axis) also exists. Whatever the mechanism, it is in these monochorionic diamniotic (MCDA) twins that TTTS commonly develops, complicating 20% overall and 10% severely (the clinical features of which are usually apparent by the mid-trimester) [3]. However, more rarely, this complication may occur in monochorionic, monoamniotic twin pregnancies when splitting of a single cellular mass occurs (days post-fertilization) (Figure 11.1.1).