*3.1.1. Number of observable gestational sacs*

The number of the gestational sacs and the number of fetal heartbeats in early multiple pregnancy scan are strongly related with chorionicity: each gestational sac will form a distinct placenta and chorion. Therefore, visualization of a single gestational sac with two visible heart beats indicates a monochorionic twin pregnancy, while the presentation of two distinctive gestational sacs implies a dichorionic pregnancy (**Picture 1**) [18]. The number of gestational sacs is the parameter with the highest accuracy to define chorionicity which is extremely close to 100% [16].
