Interpregnancy Interval and Adverse Pregnancy Outcomes: Interpregnancy Interval and Pregnancy Outcomes

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We read with interest a recent issue of Obstetrics & Gynecology that contains original research1 along with an editorial2 on interpregnancy interval and preterm birth. Dr. Klebanoff casts doubt on the causal nature of the association between short interpregnancy interval and preterm birth, citing the consistency of a lack of association identified by Hanley et al and by Ball et al.3 Both efforts employed within-mother comparisons in which each woman serves as her own control (conditional logistic regression). Our recent findings are not consistent with a lack of association despite employing similar methodology.4 That is, short interpregnancy interval of less than 6 months remained an increased risk for preterm birth even after applying conditional logistic regression. We have responded to his call for additional research.
We examined effects of short interpregnancy interval on subsequent preterm birth risks separately for women who delivered their first birth preterm or at term. Using unconditional logistic regression (across-mother comparison), a short interpregnancy interval of less than 6 months was associated with increased odds of preterm birth among women whose first birth was preterm (odds ratio [OR] [95% CI] 1.44 [1.30–1.59]) or term (OR [95% CI] 1.34 [1.28–1.40]). Results remained elevated after applying conditional logistic regression (preterm: OR [95% CI] 1.37 [1.17–1.61]; term: OR [95% CI] 1.16 [1.09–1.24]). These results are consistent with those of a recent Dutch study showing that short interpregnancy interval is associated with increased odds of preterm birth regardless of model type (unconditional or conditional).5
Next, we investigated whether risk would be altered conditional on delivery route of the first birth. Using unconditional logistic regression revealed that short interpregnancy interval of less than 6 months was associated with increased odds of preterm birth for women whose first delivery was vaginal (OR [95% CI] 1.38 [1.32–1.45]) or cesarean (OR [95% CI] 1.24 [1.13–1.36]). The results were similar when employing conditional logistic regression. That is, elevated odds of preterm birth short interpregnancy interval were observed among vaginal first deliveries (OR [95% CI] 1.21 [1.13–1.30]) and marginally so for cesarean deliveries (OR [95% CI] 1.12 [0.98–1.29]).
We share these latest findings in an effort to continue the dialog around the nature of what short interpregnancy interval may or may not be contributing to preterm birth risk.
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