Monoamniotic-monochorionic twins, aka "momo" or "mono-mono" twins, share an amniotic sac and placenta. Here's what to know about pregnancy with mono-mono twins. MoMo or mono-mono twins are ...

The inner of these membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo. The outer membrane, the chorion, contains the amnion and is part of the placenta.