The Impact of Computing Interpregnancy Intervals Without Accounting for Intervening Pregnancy Events.

Background: Short interpregnancy intervals (IPI) are associated with poor birth outcomes. Often, only livebirths are considered to estimate IPI. The objective of our work is to explore whether the associations between demographic, behavioural, and pregnancy variables and IPI change when events other than livebirth are included.

Methods: We used data from the 2006-10 and 2011-13 period of the National Survey of Family Growth (NSFG). We defined IPI using the conception date of the index pregnancy and the event date of the previous one ending in (i) livebirth; (ii) stillbirth; (iii) miscarriage; (iv) abortion; or (v) any of these events. Risk ratios (RR) were estimated for short IPI (<18 months), and demographic, pregnancy, and behavioural variables using log-linear models.

Results: When intervening events are included, the association between short IPI and its predictors vary by definition, especially for unintended versus intended pregnancies (only livebirth risk ratio [RR] 1.34, 95% confidence interval [CI] 1.2, 1.5) versus livebirth and miscarriage RR 1.14, 95% CI 1.0, 1.3) and women older than 30 vs. younger than 20 at resolution of the previous pregnancy (only livebirth RR 1.22, 95% CI 1.0, 1.5 versus livebirth and miscarriage RR 1.36, 95% CI 1.2, 1.6).

Conclusions: Including miscarriage as an intervening event in the calculation of IPI changes the association between several risk factors and short IPI. However, the association between short IPI and preterm birth does not vary when different IPI calculations are used.