Early imaging is key to detecting anomalies, some of which are unique to multiple gestations and some that also occur with singletons.
Figure 1. Thick intertwin membrane, first trimester dichorionic gestation.
Figure 2. Difficult-to-identify intertwin membrane, first trimester monochorionic gestation.
Figure 3. Second trimester twin peak sign, indicating dichorionicity.
Figure 4. Second trimester twin peak sign, indicating dichorionicity.
Figure 5. Second trimester twin peak sign, indicating dichorionicity.
Figure 6. Second trimester T-sign, indicating monochorionicity.
Figure 7. Second trimester T-sign, indicating monochorionicity.
Figure 8. Thick membrane in all 3 intertwin membranes at their juncture in the center of the uterine cavity (ipsilon sign) confirms first trimester trichorionic triplets.
Figure 9. Polyhydraminos in twin-to-twin transfusion syndrome, second trimester.
Figure 9B. The recipient fetus, left, moving freely throughout the uterine cavity. The donor fetus, right, remains in the same position.
Figure 9C. Intertwin membrane, often seen only when scanning close to the donor twin near flexed joints – neck, axillae, and hips.
Figure 10. Second trimester monoamniotic twins, umbilical cord entanglement visible on color Doppler examination.
Figure 11. 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.
Figure 11B. Axial view showing a point at which the 2 fetuses are separated.
Figure 12. Early second trimester twin reversed arterial perfusion sequence showing the amorphous acardiac fetal mass.
Figure 12B. Color Doppler examination shows umbilical cord of the amorphous acardiac mass, with blood flow.