Optimal interpregnancy interval varies by pregnancy sequence: a cohort study of singleton and twin births

Abstract Background Global preterm birth (PTB) rates remain elevated, with twin pregnancies incurring disproportionate risk. While WHO recommends a 24-month interpregnancy interval (IPI) based on singleton data, optimal IPI for pregnancy sequences involving twins remains undefined. This study aimed to identify the optimal IPI across singleton and twin pregnancy sequences. Methods This retrospective cohort study analyzed 571,791 IPI records (2004–2024) from Dongguan, China, stratifying participants by pregnancy sequence: double singleton (DS, n =562,091), twin singleton (TS, n =2652), and singleton twin (ST, n =6963). Generalized estimating equations with restricted cubic splines (GEE–RCS) modeled the non-linear IPI–PTB relationship. Optimal IPI was defined as the interval with the lowest predicted PTB probability, with uncertainty quantified by parametric bootstrapping (5000 replicates) for 95% CIs. Results Optimal IPI (i.e., the interval with the lowest predicted PTB risk in this cohort) varied: ST showed the shortest optimum at 20.9 months (95% CI 12.0–22.0), corresponding to an absolute 1.01% PTB risk reduction compared to the 24-month reference. The TS group trended toward a longer IPI, but the estimate was imprecise and could not be precisely identified, while DS aligned with guidelines (24–35 months). All sequences exhibited U-shaped patterns with significantly elevated PTB at IPIs < 6 months (adjusted OR 1.24–1.87) or >60 months (aOR 1.31–2.05). Subgroup analyses showed that prior cesarean was associated with prolonged IPIs (39.2–46.3 months), whereas younger women (< 35 years) transitioning to twins (ST) showed an association with shorter intervals (19.4 months). Conclusions Pregnancy sequence is critically associated with optimal spacing. Transitions to ST were associated with a 21-month IPI—shorter than current WHO recommendations—while post-twin pregnancies showed a trend toward longer intervals, but this estimate was imprecise and requires validation in larger cohorts. These findings support sequence-specific preconception counseling, especially in regions with rising twin birth rates.