Interpregnancy interval and adverse perinatal outcomes: A within-individual comparative method

Background and Aim: Previously observed associations between interpregnancy interval (IPI) and perinatal outcomes using a between-individual method may be confounded by unmeasured maternal factors. This study aims to examine the association between IPI and adverse perinatal outcomes using within-individual comparative analyses. Methods: We studied 10,647 individuals from the National Institute of Child Health and Human Development Consecutive Pregnancies Study in Utah with ≥3 liveborn singleton pregnancies. We matched two IPIs per individual and used conditional logistic regression to examine the association between IPI and adverse perinatal outcomes including preterm birth (PTB, <37 weeks’ gestation), small-for-gestational-age (SGA, <10th percentile of sex-specific birthweight for gestational age), low birthweight (LBW, <2,500 g), and neonatal intensive care unit (NICU) admission. Point and 95% confidence interval (CI) estimates were adjusted for factors that vary across pregnancies within individual. Results: CIs did not unequivocally support either an increase or a decrease of the odds of PTB (adjusted odds ratio (aOR) 1.31, 95% CI 0.87, 1.96), SGA (aOR 0.81, 95% CI 0.51, 1.28), LBW (aOR 1.59, 95% CI 0.90, 2.80) or NICU admission (aOR 0.96, 95% CI 0.66, 1.40) for an IPI <6 months compared to 18-23-months IPI (reference), and neither did the CIs for the aOR of IPIs of 6-11 and 12-18 months compared to the reference. In contrast, an IPI ≥24 months was associated with increased odds of LBW (aOR 1.66, 95% CI 1.03, 2.66 for 24-29 months; aOR 2.27, 95% CI 1.21, 4.29 for 30-35 months; and aOR 2.09, 95% CI 1.17, 3.72 for ≥36 months). Conclusions: Using a within-individual comparative method, we did not find evidence that short IPI compared to the recommended IPI of 18-23 months was associated with increased odds of PTB, SGA, LBW, and NICU admission. IPI ≥24 months was associated with increased odds of delivering a LBW infant.