R-MSSD and HF decreased by 8.80% (95% CI: −16.37% to −0.54%, p = 0.052) and 21.29% (95% CI: –37.75% to –0.48%, p = 0.048), respectively, in association with an IQR increase in BDNF (8.00 ng/mL). Here, BDNF is linked to hydrops fetalis.