Monochorionic twins have the highest risk of complications because the well-being of one fetus crucially depends on that of the other. One of the possible complications is known as selective Intrauterine Growth Restriction (sIUGR).
Ultrasound view of an radiofrequency ablation (RFA) needle blocking the umbilical artery of an identical twin fetus, to protect the other, healthy twin. Although the affected fetus in this case is an acardiac twin, the same procedure is utilized in extreme cases of sIUGR.
Doppler ultrasound of an acardiac twin before RFA treatment (blood flow, in red, through the umbilical artery)