Science Update: Short interval between pregnancies may increase health risks for mothers and infants

NIH-funded study suggests that maternal risks are highest for older women

Young girl kissing her pregnant mother’s stomach.
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Compared to younger mothers, mothers over 35 are at higher risk of death and serious illness if they conceive 6 months or less after the birth of a previous child, according to a study funded in part by the National Institutes of Health. For mothers of all ages, such short intervals between pregnancies were associated with higher risk for preterm birth and for having infants small for their gestation age. In contrast, an interval of 18 months between pregnancies conveyed the least risk, while an interval of 12 months was linked to only a slight increase in risk.

The study appears in JAMA Internal Medicine and was conducted by Laura Schummers, S.D., and colleagues at the Harvard T.H. Chan School of Public Health and Harvard Medical School in Boston, MA, and the University of British Columbia (UBC) in Vancouver.

NIH funding was provided by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the National Cancer Institute.


Previous studies have associated intervals of less than 18 months between giving birth and conceiving a subsequent pregnancy with higher risks of preterm birth, having an infant small for gestational age, and infant mortality, the study authors noted. Based on these studies, many healthcare practitioner groups recommend spacing pregnancies from 18 to 24 months. The researchers added that in the United States, women who begin childbearing at 30 or older tend to have shorter intervals between pregnancies compared to younger women. Pregnancy after age 35 is associated with higher risks for chromosomal abnormalities in the fetus and adverse birth outcomes like premature birth. For this reason, women may attempt shorter interpregnancy intervals to avoid the risks associated with being an older mother. The researchers conducted the current study to learn how health outcomes associated with short interpregnancy intervals might vary by age.


Researchers analyzed medical, census, and other health-related records of more than 123,000 women in British Columbia, Canada, with at least 2 consecutive pregnancies. Women over 35 who conceived within 6 months of a previous birth had a 1.2 percent risk of maternal death or severe morbidity—experiencing organ failure, needing a blood transfusion, or needing an unexpected surgical procedure after giving birth. Women over 35 who waited 18 months between pregnancies had a .5 percent risk of maternal death or severe morbidity.

For women over 35, the risk of spontaneous preterm labor after a 6-month interval was 5 percent; at 18 months it was 3.6 percent. For women from 20 to 34, the risk was 5.3 percent at 6 months and 3.6 percent at 18 months. According to the authors, all women had modest increases in risk of having an infant small for gestational age and an induced (medically indicated) preterm birth after a 6-month interval, compared to 18 months, but these risks did not vary meaningfully by age.


The researchers concluded that shorter interpregnancy intervals are associated with adverse pregnancy outcomes and that the findings provide evidence to guide clinicians counseling women who are considering spacing their pregnancies closely. The optimal interpregnancy interval of 12 to 24 months is shorter than previous studies have found, they wrote.

"Achieving that optimal one-year interval should be doable for many women and is clearly worthwhile to reduce complication risks," said the study’s senior author Wendy Norman, M.D., associate professor in the UBC Department of Family Practice.


Schummers L et al. Association of short interpregnancy interval with pregnancy outcomes according to maternal age. JAMA Internal Medicine. 2018. doi:10.1001/jamainternmed.2018.4696.

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