Sonographic assessment of chorionicity and amnionicity in twin pregnancies: how, when and why?

During the first trimester, chorionicity can be assessed as early as five weeks post-conception, but a reliable detection of amnionicity cannot be performed before the 8th week when the amnion can be clearly imaged. The late second and third trimesters twin pregnancies are a true challenge to the imaging specialist because the detection of chorionicity and amnionicity has to rely upon sonographic clues, such as placenta location, fetal gender, membrane origin, thickness, and layer. Sonographic methods used to determine chorionicity and amnionicity in twins can be extrapolated to any multifetal pregnancy, regardless of the number of fetuses present.