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

## TL;DR

This study finds that short interpregnancy intervals may not increase risks of adverse birth outcomes when comparing within the same mother.

## Contribution

The study uses within-individual comparisons to reduce confounding and finds no strong evidence linking short interpregnancy intervals to adverse outcomes.

## Key findings

- Short interpregnancy intervals (<6 months) did not show increased odds of preterm birth, SGA, LBW, or NICU admission.
- IPIs ≥24 months were associated with increased odds of low birthweight.
- Findings suggest prior associations may have been due to unmeasured maternal factors.

## Abstract

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.

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 individuals.

CIs did not unequivocally support either an increase or a decrease in 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).

Using a within‐individual comparative method, we did not find evidence that a 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 an LBW infant.

What is already known

Short and long interpregnancy intervals (IPIs) were associated with increased odds of adverse perinatal outcomes using a between‐individual method.

Short and long interpregnancy intervals (IPIs) were associated with increased odds of adverse perinatal outcomes using a between‐individual method.

What this study adds

The within‐individual comparative method showed that a short compared to recommended IPI was not associated with increased odds of small‐for‐gestational‐age, preterm birth, low birthweight (LBW), and neonatal intensive care unit admission. IPI ≥ 24 months was associated with increased odds of delivering an LBW newborn.

The within‐individual comparative method showed that a short compared to recommended IPI was not associated with increased odds of small‐for‐gestational‐age, preterm birth, low birthweight (LBW), and neonatal intensive care unit admission. IPI ≥ 24 months was associated with increased odds of delivering an LBW newborn.

Clinical implications

Previously observed associations between IPI and adverse perinatal outcomes using a between‐individual method may be due to unmeasured confounding. Intervening on a short IPI itself may not be beneficial for decreasing the prevalence of adverse perinatal outcomes.

Previously observed associations between IPI and adverse perinatal outcomes using a between‐individual method may be due to unmeasured confounding. Intervening on a short IPI itself may not be beneficial for decreasing the prevalence of adverse perinatal outcomes.