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

Importance: Interpregnancy intervals shorter than 18 months are associated with higher risks of adverse pregnancy outcomes. It is currently unknown whether short intervals are associated with increased risks among older women to the same extent as among younger women.

Objective: To evaluate whether the association between short interpregnancy (delivery to conception) interval and adverse pregnancy outcomes is modified by maternal age.

Design, Setting, and Participants: A population-based cohort study conducted in British Columbia, Canada, evaluated women with 2 or more singleton pregnancies from 2004 to 2014 with the first (index) pregnancy resulting in a live birth. Data analysis was performed from January 1 to July 20, 2018.

Main Outcomes and Measures: Risks of maternal mortality or severe morbidity (eg, mechanical ventilation, blood transfusion >3 U, intensive care unit admission, organ failure, death), small-for-gestational age (<10th birthweight percentile for gestational age and sex), fetal and infant composite outcome (stillbirth, infant death, <third birthweight percentile for gestational age and sex, delivery <28 weeks), and spontaneous and indicated preterm delivery. Risks of each outcome for 3- to 24-month interpregnancy intervals were estimated, according to maternal age at index birth (20-34 and ≥35 years). Adjusted risk ratios (aRRs) comparing predicted risks at 3-, 6-, 9-, and 12-month intervals with risks at 18-month intervals for each age group were calculated. The potential role of other factors explaining any differences (unmeasured confounding) was examined in several sensitivity analyses.

Results: Among 148 544 pregnancies, maternal mortality or severe morbidity risks were increased at 6-month compared with 18-month interpregnancy intervals for women aged 35 years or older (0.62% at 6 months vs 0.26% at 18 months; aRR, 2.39; 95% CI, 2.03-2.80), but not for women aged 20 to 34 years (0.23% at 6 months vs 0.25% at 18 months; aRR, 0.92; 95% CI, 0.83-1.02). Increased adverse fetal and infant outcome risks were more pronounced for women aged 20 to 34 years (2.0% at 6 months vs 1.4% at 18 months; aRR, 1.42; 95% CI, 1.36-1.47) than women 35 years or older (2.1% at 6 months vs 1.8% at 18 months; aRR, 1.15; 95% CI, 1.01-1.31). Risks of spontaneous preterm delivery at 6-month interpregnancy intervals were increased for women 20 to 34 years old (5.3% at 6 months vs 3.2% at 18 months; aRR, 1.65; 95% CI, 1.62-1.68) and to a lesser extent for women 35 years or older (5.0% at 6 months vs 3.6% at 18 months; aRR, 1.40; 95% CI, 1.31-1.49). Modest increases in risks of small-for-gestational age and indicated preterm delivery at short intervals did not vary meaningfully by maternal age. Sensitivity analyses suggested that observed associations were not fully explained by unmeasured confounding.

Conclusions and Relevance: The findings of this study suggest that short interpregnancy intervals are associated with increased risks for adverse pregnancy outcomes for women of all ages.