Short interpregnancy interval and pregnancy outcomes: How important is the timing of confounding variable ascertainment?

Estimation of causal effects of short interpregnancy interval on pregnancy outcomes may be confounded by time-varying factors. These confounders should be ascertained at or before delivery of the first ("index") pregnancy, but are often only measured at the subsequent pregnancy. To quantify bias induced by adjusting for time-varying confounders ascertained at the subsequent (rather than the index) pregnancy in estimated effects of short interpregnancy interval on pregnancy outcomes. We analysed linked records for births in British Columbia, Canada, 2004-2014, to women with ≥2 singleton pregnancies (n = 121 151). We used log binomial regression to compare short (<6, 6-11, 12-17 months) to 18-23-month reference intervals for 5 outcomes: perinatal mortality (stillbirth and neonatal death); small for gestational age (SGA) birth and preterm delivery (all, early, spontaneous). We calculated per cent differences between adjusted risk ratios (aRR) from two models with maternal age, low socio-economic status, body mass index, and smoking ascertained in the index pregnancy and the subsequent pregnancy. We considered relative per cent differences <5% minimal, 5%-9% modest, and ≥10% substantial. Adjustment for confounders measured at the subsequent pregnancy introduced modest bias towards the null for perinatal mortality aRRs for <6-month interpregnancy intervals [-9.7%, 95% confidence interval [CI] -15.3, -6.2). SGA aRRs were minimally biased towards the null (-1.1%, 95% CI -2.6, 0.8) for <6-month intervals. While early preterm delivery aRRs were substantially biased towards the null (-10.4%, 95% CI -14.0, -6.6) for <6-month interpregnancy intervals, bias was minimal for <6-month intervals for all preterm deliveries (-0.6%, 95% CI -2.0, 0.8) and spontaneous preterm deliveries (-1.3%, 95% CI -3.1, 0.1). For all outcomes, bias was attenuated and minimal for 6-11-month and 12-17-month interpregnancy intervals. These findings suggest that maternally linked pregnancy data may not be needed for appropriate confounder adjustment when studying the effects of short interpregnancy interval on pregnancy outcomes.

Schummers L ，Hutcheon JA ，Norman WV ，Liauw J ，Bolatova T ，Ahrens KA ... -  《-》

Association of Short Interpregnancy Interval With Pregnancy Outcomes According to Maternal Age.

Schummers L ，Hutcheon JA ，Hernandez-Diaz S ，Williams PL ，Hacker MR ，VanderWeele TJ ，Norman WV ... -  《-》

Interpregnancy Interval and Adverse Pregnancy Outcomes: An Analysis of Successive Pregnancies.

Hanley GE ，Hutcheon JA ，Kinniburgh BA ，Lee L ... -  《-》

The influence of interpregnancy interval on the subsequent risk of stillbirth and early neonatal death.

Stephansson O ，Dickman PW ，Cnattingius S 《OBSTETRICS AND GYNECOLOGY》

Short interpregnancy intervals and adverse pregnancy outcomes by maternal age in the United States.

The purpose of the article was to examine the association between short interpregnancy intervals and adverse outcomes by maternal age among U.S. women. Using publicly available natality files for 2013-2016 singleton births, we compared the risks of preterm birth, gestational diabetes, gestational hypertension, and maternal morbidity (delivery-related complications) for less than 6-month, 6 to 11-month, and 12 to 17-month to 18- to 23-month interpregnancy intervals, overall and by maternal age. Models adjusted for maternal demographics, conditions, and behaviors. Among 2,365,219 births, adjusted risk ratios (aRR) for preterm birth overall for intervals less than 6, 6-11, and 12-17 months were 1.62 (95% confidence interval: 1.60, 1.65), 1.16 (1.15, 1.18), and 1.03 (1.02, 1.05), respectively, compared with 18-23 months. Intervals less than 6, 6-11, and 12-17 months were significantly protective overall for gestational diabetes (aRR range: 0.89-0.98), gestational hypertension (aRR range: 0.93-0.95), and maternal morbidity (aRR range: 0.93-1.08). All aRRs attenuated or remained flat with increasing maternal age. Interpregnancy intervals less than 18 months showed different patterns of association for preterm birth compared with maternal outcomes, overall and across age. This suggests that increasing maternal age may have discordant effects on associations between short interpregnancy interval and adverse perinatal and maternal outcomes.

Haight SC ，Hogue CJ ，Raskind-Hood CL ，Ahrens KA ... -  《-》