NPPB and hydrops fetalis: There were 7496 patients in the study: 2620 (34%) had HeFREF, 2163 (28%) had HeFPEF (NT‐proBNP ≥400 ng/L), 1065 (14%) had HeFPEF (NT‐proBNP between 125 & 399 ng/L), 861 (11%) did not fulfil criteria for HF (‘controls’), leaving 787 (10%) with diagnostic uncertainty (normal LV systolic function but no NT‐proBNP result, ‘No NT‐proBNP’) (Supporting Information, FigureS1).