Bart's hydrops fetalis is lethal because –
`In the foetus excess, gamma globin chains form tetramers (1Ib Barts) with such high affinity for oxygen that they deliver almost no oxygen to tissues' - Robbins

Clinically this condition manifests as hydrops fetalis and fetus shows features similar to those of erythroblastosis fetalis → Severe pallor, Generalized edema, Massive hepatosplenomegaly
Hb Bart's cannot bind oxygen
The excess a – globin form insoluble precipitates
Hb Bart's cannot release oxygen to fetal tissues
Microcytic red cells become trapped in the placental