NPPA and hydrops fetalis: Progressive increases in serum levels of brain (or B-type) natriuretic peptide (BNP) (Fig. 1e), elevated mRNA expression levels of established markers Nppb or Nppa (Extended Data Fig. 2c) and increased cardiomyocyte hypertrophy (Extended Data Fig. 2d) confirmed that both PAB and AOB resulted in terminal HF that proceeded through an intermediate compensatory state.