The dichorionic intertwin membrane is very thick and easy to identify in the first trimester.
The monochorionic intertwin membrane is difficult to identify in the late first trimester.
Second trimester twin peak sign, indicating dichorionicity
Second trimester twin peak sign, indicating dichorionicity
Second trimester twin peak sign, indicating dichorionicity
Second-trimester T-sign, indicating monochrionicity
Second-trimester T-sign indicating monochorionicity
Thick membrane in all 3 intertwin membranes at their juncture in the center of the uterine cavity (ipsilon sign) confirms the first-trimester trichorionic triplets
Polyhydramnios in the recipient twin's sac in a case of twin-to-twin transfusion syndrome, second trimester
The recipient fetus, left, moving freely throughout the uterine cavity. The donor fetus, right, remains in the same position.
Intertwin membrane, often seen only when scanning close to the donor twin near flexed joints - neck, axillae, and hips.
Second trimester monoamniotic twins, umbilical cord entanglement visible on color Doppler examination
First-trimester conjoined twins. Sagittal view shows 2 cephalic poles, right. Note that the upper cephalic pole could be misinterpreted as a large omphalocele of a single fetus.
Axial view showing a point at which the 2 fetuses are separated.
Early second-trimester twin reversed arterial perfusion sequence showing the amorphous acardiac fetal mass
Color Doppler examination shows umbilical cord of the amorphous acardiac mass, with blood flow