Twelve to 18 months seems to be the ideal length of time between giving birth and getting pregnant again, according to research from the University of British Columbia and the Harvard T.H. Chan School of Public Health. In a study published in JAMA Internal Medicine (“Association of short interpregnancy interval with pregnancy outcomes according to maternal age”), researchers found that getting pregnant less than 12 months after delivery is associated with risks for women of all ages. Risks to the mother were found only for women over the age of 35, while risks to the infant were found for all women, but were greatest for women between the ages of 20 and 34.

The study is the most extensive evaluation of how the role of pregnancy spacing could be impacted by maternal age. It is also the first investigation of pregnancy spacing and maternal mortality or severe morbidity in a high-income country. The authors note that the findings about older women are particularly important, as they tend to more closely space their pregnancies, often intentionally.

Researchers examined the relationship between risks for mothers and babies associated with pregnancy spacing among 148 544 pregnancies in BC. Data were collected from birth records, billing codes, hospitalization data, prescription data for infertility information, and census records. Among women over 35 who conceived 6 months after a previous birth, researchers found a 1.2% risk of maternal mortality or severe morbidity. Waiting 18 months between pregnancies reduced the risk to 0.5%. For younger women, researchers found an 8.5% risk of spontaneous preterm birth for pregnancies spaced at 6 months. For younger women who waited 18 months between pregnancies the risk dropped to 3.7%. Among older women, the risk of spontaneous preterm labor was about 6% at the 6-month interval, compared to 3.4% at the 18-month interval. Although the causes of poor pregnancy outcomes at short intervals among older and younger women were not examined in this study, the findings suggest different risk profiles for each age group.

The authors reflect that whether the elevated risks are due to the body not having time to recover if women conceive soon after delivering or to factors associated with unplanned pregnancies, such as inadequate prenatal care, the recommendation might be the same: improve access to postpartum contraception or abstain from unprotected sexual intercourse with a male partner following a birth.

The study was coauthored by Laura Schummers, SD, Jennifer A. Hutcheon, PhD, Sonia Hernandez-Diaz, DrPH, Paige L. Williams, PhD, Michele R Hacker, SD, Tyler J. VanderWeele, PhD, and Wendy V. Norman, MD. It is available at https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/2708196 [9].