In NWS, MPO activity (↑43.48%, p < 0.0001; ↑29.41%, p < 0.0001, respectively) and the concentration of peroxynitrite (↑60.29%, p < 0.0001; ↑41%, p < 0.0001, respectively) and nitrotyrosine (↑68.52%, p = 0.0011; ↑53.78%, p < 0.0001, respectively) were significantly higher in the group of HF patients with NS and HS compared to the controls, while the content of S-nitrosothiols (↑75.61%, p = 0.1054) was considerably higher only in HF patients with HS compared to the control group. Here, MPO is linked to hydrops fetalis.