An acardiac twin in a multiple pregnancy initially develops normally and is a specific complication of monozygous multiple pregnancies. Development results from arterio-arterial and veno-venous anastomoses leading to predominance of one of the twins. The haemodynamic abnormalities in the dominated twin lead to the disappearance of the heart and major morphologic malformations. Outcome is generally unfavourable in 50% of the pregnancies with an acardiac twin. Complications in the healthy twin include heart failure, then hydramnios, and finally premature delivery which is the cause of most of the deaths. Proposed treatment currently relies on treating the heart failure in the healthy twin or interrupting vascularization between the two twins leading to in utero death of the acardiac twin. Antenatal diagnosis is made by echography and is useful for evaluating the prognosis for the pregnancy according to the growth of the acardiac twin.