When blood circulations of twins are connected, a fetus without a functioning heart (acardius) can survive in utero. Acardius can either be a completely amorphous mass or a hydropic object with schematic limbs, trunk and head, but vital organs are not developed (lungs, liver...). Yhe heart is either missing completely or is seriously malformed (univentricular heart etc.). The heart of the healthy twin works for them both (pump twin). This heart hypertrophies, and the fetus may die of congestive heart failure. The acardius has reversed blood circulation (twin reversed arterial perfusion).