Mirror syndrome, or Ballantyne syndrome (BS), was described for the first time in 1892 by John W. Ballantyne. It is classically defined as triple oedema: association of foetal, placental and maternal oedema. In this syndrome, a pregnant woman with hydrops fetalis reflects (as in a mirror) the signs present in the foetus and develops symptoms similar to those displayed by the foetus. Mirror syndrome is not a widely described disease, its prevalence is unknown and there are relatively few literature reports of the condition. It coexists with various pathologies of pregnancy and structural anomalies of the foetus and placenta; it may occur after viral infections during pregnancy. The pathogenesis of Ballantyne syndrome remains unexplained and there is a wide variety of clinical signs. There exist theories for the pathomechanism of the syndrome, but none of them is fully satisfactory. Mirror syndrome is potentially life-threatening and is associated with increased foetal mortality and maternal morbidity. The treatment of choice for mirror syndrome in the mother is the treatment of oedema in the foetus, as the aetiology of the condition suggests. In mirror syndrome, the prognosis for the foetus is unfavourable and many cases end with intrauterine death. The treatment of hydrops fetalis, regardless of its cause, often leads to the resolution of symptoms in the mother, and, at the same time, better prognosis for the foetus.