Circulating Endothelial Nitric Oxide Synthase on the First Postpartum Day: An Exploratory Cross-Sectional Study of Its Association with Maternal Age, Redox-Relevant Haematological and Lipid Markers, and Postnatal Depression Screening

(1) Background: Endothelial nitric oxide synthase (eNOS/NOS3), the main source of vascular nitric oxide (NO), generates superoxide instead of NO when oxidative stress uncouples it. Circulating eNOS in the early puerperium is uncharacterised. (2) Methods:In a single-centre cross-sectional study (Timișoara, Romania, July 2022), 40 mother–newborn dyads were evaluated. Maternal serum eNOS protein was quantified by ELISA on postpartum day 1, alongside pre-delivery laboratory data and Edinburgh Postnatal Depression Scale (EPDS) screening. Analyses (Spearman correlation, linear regression, prevalence odds) were unadjusted for multiplicity; only correlations of |ρ| ≥ 0.44 were detectable at 80% power. (3) Results:Median serum eNOS was 8.997 ng/mL (interquartile range 4.091–27.985). eNOS correlated inversely with total and LDL cholesterol (ρ = −0.368, p = 0.027; ρ = −0.412, p = 0.013) and positively with erythrocyte count (ρ = 0.326, p = 0.046). The association with maternal age was similar in magnitude but non-significant (ρ = −0.303, p = 0.061). Multivariable models were not significant, and eNOS was unrelated to EPDS score (ρ = 0.111, p = 0.512). (4) Conclusions:Lower circulating eNOS protein, not enzyme activity, accompanied a less favourable lipid profile, but not depressive symptoms. These single-centre Romanian findings are hypothesis-generating and should not be generalised.