New mothers who become pregnant less than a year after giving birth could be putting themselves and their unborn child at increased risk, a study has suggested.
Researchers who looked into nearly 150,000 births in Canada said those who waited 12 to 18 months to conceive after having a baby reduced the risk of short- and long-term damage to both the mother and child's health.
The study, by the University of British Columbia (UBC) and the Harvard TH Chan School of Public Health, found a short gap between pregnancies affected mothers aged over 35, while risks to the infant were found for all women - particularly for those aged 20 to 34.
Laura Schummers, a post-doctoral fellow at the UBC, said: "Our study found increased risks to both mother and infant when pregnancies are closely spaced, including for women older than 35.
"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 described by the authors as 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, the authors said.
Among women over 35 who conceived six months after a previous birth, the researchers found a 1.2 per cent risk of maternal mortality or severe morbidity. Waiting 18 months between pregnancies, however, reduced the risk to 0.5 per cent, the authors said.
For younger women, the researchers found an 8.5 per cent risk 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.
Senior author Dr Wendy Norman, UBC associate professor, said: "Achieving that optimal one-year interval should be doable for many women, and is clearly worthwhile to reduce complication risks."