As shown on ultrasound, monochorionic twins will have a single placenta, with the interfetal septum forming a “T’ as it meets the base. The absence of chorion in the septum makes the septum thin (only two layers) or ‘very hard to see,’ often giving the false impression of monochorionic monoamnionic twins (see Figure #1).
In contrast, separate placental masses and the ‘lambda’ or ‘twin peak’ sign (see Figure #2) in the 1st trimester both indicate dichorionic placentation, not at risk for TTTS.
Figure 1. The arrow on the left points to a thin (< 1.5 mm) interfetal membrane septum in monochorionic twins, which forms a ‘T’ at the base, and is ‘hard to see’ in some areas (right arrow). These twins are ‘at risk’ for TTTS.
Figure 2. The arrows point to the interfetal membrane septum, which contains a triangular projection of chorionic villi at the septum base. This early sign ensures a dichorionic pregnancy.