Current ultrasound definition of twin–twin transfusion syndrome

Several important achievements have been made in the last decade in defining TTTS. First, for the reasons mentioned above, the condition requires a sonographic diagnosis. 

This is important as it establishes ultrasound as the sole reliable diagnostic tool for the condition. As a corollary, the diagnosis of TTTS in patients with monochorionic twin pregnancies with an adverse pregnancy outcome or with significant size discordance between the twins but without prenatal sonographic diagnosis of TTTS lacks definite proof of the condition and may only rely on circumstantial evidence. 

Secondly, there has been a gradual acceptance of the sonographic criteria by which TTTS is defined: namely, the presence of polyhydramnios, defined as a maximum vertical pocket of ?8 cm in one sac, and oligohydramnios, defined as a vertical pocket of ?2 cm in the other sac. Finally, some sonographic parameters are no longer part of the definition – namely, abnormal Doppler studies or growth discordance – as will be explained below.