Short Interdelivery Interval and Neonatal Acid–Base Status, Postpartum Anemia, and Postnatal Depression: A Retrospective Cohort Study with Within-Mother Sensitivity Analysis

Background/Objectives: Short interdelivery interval (IDI) is associated with adverse perinatal outcomes, but its relationship with objective neonatal acid–base markers, postpartum anemia, and early postnatal depression remains poorly characterized. We examined IDI < 24 months across four pre-specified outcome domains: neonatal acid–base status, maternal composite morbidity, postpartum anemia, and early postnatal depression. Methods: This single-center retrospective cohort study included 851 women with two consecutive singleton live births at Ankara Etlik City Hospital between 2023 and 2025. Women were classified by IDI as short (<24 months; n = 635) or standard (≥24 months; n = 216). Multivariable logistic regression provided the primary inference. Sensitivity analyses included inverse probability of treatment weighting (IPTW), restricted cubic splines (4 df), within-mother paired analysis, multiple imputation by chained equations (MICE × 20), Firth penalization, E-values, and Benjamini–Hochberg false discovery rate (FDR) correction across 13 outcomes. Results: After FDR correction, short IDI was associated with postpartum anemia (adjusted odds ratio [aOR] 1.84, 95% confidence interval (CI) 1.26–2.68; q = 0.022) and with an Edinburgh Postnatal Depression Scale score ≥13 (aOR 1.93, 95% CI 1.20–3.10; q = 0.042); umbilical-artery pH < 7.10 did not cross the FDR threshold (aOR 2.37, 1.17–4.82; q = 0.075) and is reported as an exploratory, hypothesis-generating signal. Postpartum anemia did not mediate the IDI–depression association (proportion mediated 0.6%, 95% CI −13.1% to +13.5%). Conclusions: Short IDI was associated with maternal hematologic and psychosocial signals; the IDI–depression link appears non-hematologic. Neonatal acid–base findings did not meet the FDR threshold and are exploratory. Multicenter validation is warranted.