Figure 4. This is a placenta from a TTTS pregnancy that was lost at 20 weeks' gestation before treatment could be started. The twins had all three types of anastomoses between them (artery-to-artery, artery-to-vein, vein-to-vein). The vascular equator (arrows) divided the placenta into two portions that were significantly asymmetrical (95% vs. 5%). The twin with the smaller share (donor) has an umbilical cord attachment in the membranes (velamentous) outside the body of the placenta, while the other twin's cord inserted into the center of the placenta. The donor's placenta share was too small to provide its nutrients beyond 20 weeks and was the cause of its death. The larger twin then passed away from an acute transfusion into the other twin through the large artery-to-artery connection seen near the tip of the upper arrow. (Photo courtesy of Dr. Kurt Benirschke)