Interpregnancy Interval and Adverse Perinatal Outcomes: A Within-Woman Comparison Approach
Introduction: Short interpregnancy interval (IPI) examined using between-women comparisons design, is associated with increased risks of adverse perinatal outcomes in the subsequent pregnancies. We aimed to examine the association between IPI and odds of perinatal outcomes in the subsequent pregnancy utilizing within-woman comparison design.
Methods: We studied 10,647 women from the NICHD Consecutive Pregnancy Study in Utah (2002-2010) with ≥3 liveborn singleton pregnancies. IPI was categorized as 0-5, 6-11, 12-17, 18-23 (reference), and ≥24 months. Perinatal outcomes were preterm birth (PTB, delivery at <37 weeks’ of gestation), small for gestational age (SGA, <10th percentile of sex-specific birth weight for gestational age), and low birth weight (LBW, birthweight <2,500 grams). We used matched design (matching two IPIs per mother) and used conditional logistic regression to perform within-woman comparisons to obtain adjusted odds ratios (OR) and 95% confidence intervals (CI). Estimates were adjusted for age, marital status, insurance, prepregnancy BMI, smoking and chronic conditions.
Results: Most of the women were white (89%), young (mean age 27 years), married (92%), with private insurance (75%). PTB was not associated with short (OR 0-5 vs 18-23=1.25, 95% CI: 0.84, 1.87) or long (OR ≥24 vs 18-23=1.03, 95% CI: 0.78, 1.35) IPI. Similarly, SGA was not associated with short (OR 0-5 vs 18-23 = 0.82, 95% CI: 0.52, 1.28) or long (OR ≥24 vs 18-23=1.09, 95% CI: 0.81, 1.47) IPI. No association was found with short IPI and LBW. The long IPI was associated with increased LBW odds (OR ≥24 vs 18-23=1.73, 95% CI: 1.18, 2.54).
Conclusion: No associations were found with short IPI categories and PTB, SGA and LBW. The IPI ≥24 months was associated with almost twice increased odds of LBW compared to reference IPI. Previously observed associations between short IPI categories and adverse perinatal outcomes using between-women design may be due to unmeasured confounding by maternal factors.