Monoamniotic twins result following splitting of the embryoblast from days 8 to 12 post conception. Division after this time results in conjoined twins. They share not only the chorion (the outer membrane) but also the amnion (the inner membrane) and thus are in the same gestational sac. Monoamniotic twins may suffer from multiple problems including twin-twin transfusion (although less commonly and at a lesser severity than monochorionic diamniotic twins). Entanglement of the cords occurs because of the increased mobility of the twins in the second trimester. In the third trimester, the reduced space is usually no longer sufficient to allow the twins to move around. In spite of apparent cord compression with absent end-diastolic velocities (AEDV), some fetuses have grown appropriately. The significance of AEDV in monoamniotic twins may thus be less predictive than in singletons. There is an increased risk of congenital anomalies (15-20%). The overall mortality of monoamniotic twins can be as high as 50-60%.
Counting twins with different chorionicity by counting the number of gestational sacs is easier in the first trimester when thick layers of tissue separate the sacs. The correct determination of amnionicity is not quite so easy. The amniotic membrane is very thin, and unless the ultrasound beam is perpendicular, it may be difficult to discern. A simple trick that is convincing when present is to roll the patient to the side and observe the passive motion of the embryos. If they both gravitate to the bottom of the gestational sac no matter what decubitus position, the suspicion of monoamniotic twins is high.
Monoamniotic twins can easily be confused with monochorionic diamniotic twins, especially when there is twin-twin transfusion and one of the twins is stuck. A careful search for a membrane, in particular between the limbs and the body, is the only way to ascertain the diagnosis.
No dividing membrane