Occlusion of umbilical artery in acardiac, acephalic twin. In the acardiac, acephalic twin malformation the normal co-twin is put at risk because of the extra cardiac work-load. Surgical procedures may be hazardous to the mother. We describe a novel approach--the insertion of a helical metal coil to induce thrombosis in the umbilical artery of the acardiac twin--which immediately interrupted flow. The co-twin was delivered at 39 weeks and his neonatal course has been normal. 