REN and hydrops fetalis: Among these indices, TP is considered as all spectra of the frequencies; LF, LF%, and nLF activity indicate the sympathetic effects; HF, HF%, and nHF activity have been related to parasympathetic nervous activity, which represents the vagal-mediated modulation of heart rate; LF/HF ratio is an index of sympathetic to parasympathetic balance; and VLF is considered to reflect vasomotor function, the renin–angiotensin–aldosterone system, and the cardiac response toward external stress [29,30,31,32].