Adverse pregnancy outcomes are known to be associated with getting pregnant less than 18 months after giving birth. But are such short interpregnancy intervals even riskier in mothers who are older? Results of a population-based study provide new perspectives on the interaction between spacing of pregnancies, maternal age, and the health of mothers and their offspring.
Published in JAMA, the findings, according to the researchers, represent “robust evidence to guide clinicians counseling women considering short interpregnancy intervals.” Unlike the methodology used in previous studies in this area, the authors said, their database allowed them to control for stillbirth and neonatal loss in the index birth and to accurately measure intervals between pregnancies.
Conducted in British Columbia, Canada, the cohort study focused on analysis of data from nearly 150,000 pregnancies. The participants were women with two or more singleton pregnancies from 2004 to 2014 and the data were analyzed from January 1 to July 20, 2018.
The authors looked at risks of maternal mortality or severe morbidity (e.g., mechanical ventilation, blood transfusion > 3 U, intensive care unit admission, organ failure, death), small-for-gestational age [SGA], fetal and infant composite outcome (stillbirth, infant death < third birthweight percentile for gestational age and sex, delivery < 28 weeks), and spontaneous and indicated preterm birth (PTB). Risks of each outcome for 3- to 24-month interpregnancy intervals were estimated, according to maternal age at birth (20-34 and ≥ 35 years). For each age group, the researchers then calculated adjusted risk ratios (aRRs) comparing predicted risks at 3- 6-, 9- and 12-month intervals with risks at 18-month intervals. Sensitivity analyses also were performed to assess the potential role of other factors in explaining any differences.
In women aged ≥ 35, the researchers found that maternal mortality or severe morbidity risks were increased at 6-month versus 18-month interpregnancy intervals (0.62% at 6 months vs. 0.26% at 18 months; aRR 2.35; 95% CI, 2.03-2.80). However, the same was not the case in women aged 20 to 34 (0.23% at 6 months vs. 0.25% at 18 months; aRR 0.92; 95% CI, 0.83-1.02).
In women aged 20 to 34 years versus those aged ≥ 35, increased adverse fetal and infant outcome risks were more pronounced (20.0% at 6 months vs. 14% at 18 months; aRR 1.42; 95% CI 1.36-1.47 and 2.1% at 6 months vs. 1.8% at 18 months; aRR 1.15; 95% CI 1.01-1.31, respectively). Risks of spontaneous PTB at 6-month interpregnancy intervals were increased in both age groups, but to a lesser extent in the older women (5.3% at 6 months vs 3.2% at 18 months; aRR 1.65; 95% CI 1.62-1.68 and 5.0% at 6 months vs 3.6% at 18 months; aRR 1.40; 95% CI 1.31-1.49, respectively). Maternal age did not significantly affect the modest increases in risks of SGA and indicated PTB with short interpregnancy intervals. Sensitivity analyses suggested that observed associations were not fully explained by unmeasured confounding.
The authors said their findings, “suggest that the optimal interpregnancy interval is closer to 18 months, with a range of 12 to 24 months having risks that generally are comparable to the nadir at 18 months.” In the case of women aged ≥ 35, they believe that counseling to encourage an interpregnancy interval of 12 or 18 months would help reduce risks. But they acknowledged that in this population, a subsequent pregnancy in a short window of time after the first one often is planned.