Twelve to 18 months seems to be the ideal length of time between giving birth and getting pregnant again, according to new research from the University of British Columbia and the Harvard T.H. Chan School of Public Health.

In a study published Oct. 29 in JAMA Internal Medicine, 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 20-34.

"Our study found increased risks to both mother and infant when pregnancies are closely spaced, including for women older than 35," said the study's lead author Laura Schummers, a postdoctoral fellow in the UBC department of family practice who carried out the study as part of her dissertation at the Harvard T.H. Chan School of Public Health. "The findings for older women are particularly important, as older women tend to more closely space their pregnancies and often do so intentionally."

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--rare but life-threatening complications of pregnancy, labour and delivery--in a high-income country.

For the study, researchers examined the relationship between risks for mothers and babies associated with pregnancy spacing among 148,544 pregnancies in B.C. The data was pulled together from birth records, billing codes, hospitalization data, prescription data for infertility information, and census records.

Among women over 35 who conceived six months after a previous birth, the researchers found a 1.2 per cent risk (12 cases per 1,000 pregnancies) of maternal mortality or severe morbidity. Waiting 18 months between pregnancies, however, reduced the risk to 0.5 per cent (five cases per 1,000 pregnancies).

For younger women, the researchers found an 8.5 per cent risk (85 cases per 1,000 pregnancies) of spontaneous preterm birth--delivery before 37 weeks of pregnancy after labour that started on its own, for pregnancies spaced at six months. For younger women who waited 18 months between pregnancies, however, the risk dropped to 3.7 per cent (37 cases per 1,000 pregnancies).

Among older women, the risk of spontaneous preterm labour was about six per cent (60 cases per 1,000 pregnancies) at the six-month interval, compared to 3.4 per cent (34 cases per 1,000 pregnancies) 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.

"Short pregnancy spacing might reflect unplanned pregnancies, particularly among young women," said Dr. Sonia Hernandez-Diaz, professor of epidemiology at the Harvard T.H. Chan School of Public Health. "Whether the elevated risks are due to our bodies not having time to recover if we conceive soon after delivering or to factors associated with unplanned pregnancies, like 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."

Senior author Dr. Wendy Norman, associate professor in the UBC department of family practice, said these findings of a shorter optimal interval are encouraging for women over 35 who are planning their families.

"Older mothers for the first time have excellent evidence to guide the spacing of their children," said Norman. "Achieving that optimal one-year interval should be doable for many women, and is clearly worthwhile to reduce complication risks."

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The study was co-authored by Jennifer Hutcheon at the UBC department of obstetrics and gynaecology, and Paige Williams, Michele Hacker and Tyler VanderWeele at the Harvard T.H. Chan School of Public Health Department of Epidemiology.