Shorter Interpregnancy Interval Risks Vary by Maternal Age

Miriam E. Tucker

October 30, 2018

Short interpregnancy intervals are associated with increased maternal risks for older women, and greater fetal and infant risks for younger women, new research suggests.

Findings from a large Canadian population-based cohort study were published online October 29 in JAMA Internal Medicine by Laura Schummers, SD, Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, and colleagues.

The study included 148,544 pregnancies among 123,122 women. Maternal mortality or severe morbidity risks were increased at short (< 12 months) interpregnancy intervals among those 35 years or older but not for women aged 20 to 34 years. In contrast, increased risks for adverse fetal and infant outcomes and spontaneous preterm delivery were higher with shorter pregnancy intervals for women aged 20 to 34 years than for those 35 years and older.

The findings provide guidance for older women who are weighing the competing risks of increasing maternal age — including infertility, chromosomal anomalies, and adverse birth outcomes — with those of shorter intervals between pregnancies, Schummers and colleagues say.  

Current clinical and public health recommendations suggest a minimum interpregnancy interval of 18 months, and some suggest a range from 18 to 60 months. However, the authors say, the new findings suggest that the optimal interpregnancy interval is closer to 18 months, and a range of 12 to 24 months has risks that are generally comparable to the nadir at 18 months.

"We present what we believe to be new, robust evidence to guide clinicians counseling women considering short interpregnancy intervals," Schummers and colleagues write.  

In an accompanying editorial, Stephanie B. Teal, MD, and Jeanelle Sheeder, PhD, Division of Family Planning, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora, note that the mean age at first birth has risen from 24.2 years in 1990 to 26.3 years in 2014. And those who desire a second child will often attempt to conceive again soon after.

"Older women need to weigh the risks to their own health and their infants' health against the risks of not achieving the number of children they want. Simply telling older women to delay conception is not likely to improve health outcomes, as women are aware of their 'biological clocks' and many will value their desire for another child over their physician's warnings," the editorialists say.

Rather, they advise, "Clinicians should use patient-centered counseling and shared decision-making strategies that respect women's desires for pregnancy, possibly at short intervals in women 35 years or older, and adequately discuss fetal, infant, and maternal risks in this context."

For older women who desire another pregnancy, Teal and Sheeder recommend that clinicians "carefully evaluate and manage pre-existing maternal health indices, such as return to normal weight and normalization of blood pressure and glucose control after the index pregnancy."

And for younger women, "clinicians should continue to emphasize the improved outcomes with adequate birth spacing and promote postpartum contraception to help their patients achieve optimal maternal and infant outcomes and fertility goals."

Risks From Short Intervals Vary With Maternal Age

The investigators evaluated women with two or more singleton pregnancies from 2004 to 2014 with the first resulting in live births. They defined interpregnancy interval as the time in months between the index birth and conception of the subsequent pregnancy.

Overall, interpregnancy intervals of fewer than 12 months were associated with significant increases in maternal mortality or severe morbidity, adverse fetal and infant outcomes, and spontaneous preterm delivery with adjusted relative risks (aRR) of 1.18, 1.36, and 1.59, respectively, for 6-month versus 18-month intervals.

Predicted absolute risks for intervals of 6 months versus 18 months were 0.30% versus 0.25% for maternal mortality/severe morbidity, 2.0% versus 1.5% for adverse fetal/infant outcomes, and 5.3% versus 3.3% for spontaneous preterm delivery.

After stratification by maternal age category, the aRR for maternal mortality or severe morbidity for those aged 35 and older at index birth was a significant 2.39 for 6-month versus 18-month interpregnancy intervals, with absolute predicted risks of 0.62% versus 0.26%, respectively. However, that risk was not elevated for women aged 20 to 34 years at index birth (aRR, 0.92; 0.23% vs 0.25%, respectively). 

In contrast, the increased risk for spontaneous preterm delivery at short interpregnancy intervals was more pronounced in younger women (aRR, 1.65; absolute risks, 5.3% at 6 months vs 3.2% at 18 months, respectively) than older women (aRR, 1.40; 5.0% vs 3.6%), although the risk remained significant for all ages.

Adverse fetal and infant outcomes were also more pronounced for women aged 20 to 34 years (aRR, 1.42; predicted risk, 2.0% at 6 months vs 1.4% at 18 months) compared with those aged 35 years and older (aRR, 1.15; 2.1% vs 1.8%), although again, all remained significant.

There were modestly increased risks for small-for-gestational-age and indicated preterm delivery at shorter intervals, but those did not vary significantly by maternal age. Results remained similar in sensitivity analyses.

The study was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Canadian Institutes for Health Research, Public Health Agency of Canada Family Planning Public Health Chair Seed Grant, and Michael Smith Foundation for Health Research.

The authors and editorialists have disclosed no relevant financial relationships.

JAMA Int Med. Published online October 29, 2018. Abstract, Commentary

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