Impact of Short and Long Interpregnancy Intervals on Neonatal Outcomes: A Multiclassification Cohort Analysis

Introduction: Interpregnancy interval (IPI) plays a critical role in neonatal health, yet optimal spacing remains controversial. This study assessed neonatal outcomes across short and long IPI using three complementary classification approaches to identify consistent patterns of risk. Materials and Methods: In this retrospective cohort study, medical records of 1194 women with a prior live birth who delivered singleton pregnancies in 2024 at a tertiary referral center were analyzed. IPI was calculated as the delivery-to-conception interval (LMP + 14 days). Three IPI classification systems were applied: (1) classical cut-offs (<6, 6-11, 12-23, 24-59, and ≥60 months), (2) quartiles, and (3) tertiles. Primary outcomes included preterm birth, low birth weight (LBW), and NICU admission. Multivariable logistic regression models adjusted for maternal age, gravidity, and previous cesarean delivery. Results: Short IPI (6-11 months) demonstrated the highest NICU admission rates (29.4%). Very long IPI (≥60 months) showed the highest prevalence of LBW (16.6%). Multivariable regression analysis revealed that intervals ≥ 24 months were independently protective against preterm birth (24-59 months: aOR 0.48, p = 0.002; ≥60 months: aOR 0.58, p = 0.042), while maternal age increased preterm birth risk by 7% per year. Short IPI (6-11 months) and very long IPI (≥60 months) independently increased NICU admission risk (aOR 2.29, p = 0.002 and aOR 1.61, p = 0.036, respectively). Previous cesarean delivery was an independent predictor of NICU admission (aOR 1.35; p = 0.048). Conclusions: Short and very long IPIs are associated with increased neonatal morbidity, particularly NICU admission, while the apparent preterm risk in long intervals is largely mediated by maternal age. Once adjusted, IPIs exceeding 24 months demonstrate protective effects against preterm birth. However, the rising trend toward LBW and NICU admission in intervals beyond 5 years suggests that birth-spacing counseling targeting an optimal window of 18-24 months provides the best balance in minimizing competing neonatal risks.