Gaining too much weight during pregnancy raises complication risks

A new study of more than one million births led by McMaster University researchers has found that gaining too much weight during pregnancy, regardless of a person's starting weight, is linked to a higher risk of life-threatening maternal complications.

Current pregnancy weight-gain recommendations were developed primarily to improve outcomes for babies, such as healthy birth weight and reducing the risk of preterm birth. The study, published in JAMA Network Open on Oct. 22, 2026, suggests the recommendations should also be evaluated through the lens of maternal health, including the risk of life-threatening complications such as bleeding, pre-eclampsia and ICU admission during childbirth.

"We've long known that a person's weight before pregnancy is a risk factor. We wanted to know whether the amount of weight gained during pregnancy also influences the risk of complications," says lead author Sahar Khademioore, a Health Research Methodology PhD candidate at McMaster. "Because weight gain is something that can potentially be addressed, understanding how it relates to severe complications gives us an opportunity to better support maternal health."

Of the more than one million Ontario pregnancies studied using the BORN Ontario birth registry:

• 53 per cent gained more weight than recommended.
• 19 per cent gained less than recommended.
• More than 15,000 severe maternal morbidity cases were identified.
• Excessive weight gain increased risk across every body mass index (BMI) category.

People with pre-existing conditions such as diabetes, asthma and high blood pressure faced roughly double the risk of severe maternal complications, with risks increasing further when weight gain fell outside recommended ranges. Researchers also found that gaining less than the recommended amount of weight was associated with higher risks of severe maternal outcomes in some BMI groups.

More than half of pregnancies in our study involved weight gain above current recommendations, underscoring how challenging healthy weight gain during pregnancy can be. Our findings provide important evidence for clinicians and policymakers, but supporting healthy weight gain will require more than individual counselling. We also need broader, population-level strategies that create supportive environments for healthy pregnancies."

Laura Anderson, senior author, associate professor with McMaster's Department of Health Research Methods, Evidence, and Impact

The study is part of the SERENE Project, a research effort in partnership with a lived experience advisory network that is committed to exploring why and when severe maternal morbidity happens, which people are most affected, what are the long-term physical, social and mental health implications, and how severe maternal morbidity and its negative effects can be reduced. 

Funding for this study was provided by the Juravinski Research Institute, a partnership with Hamilton Health Sciences, McMaster University and St. Joseph's Healthcare Hamilton established through the generosity of the late Margaret and Charles Juravinski, as part of the SERENE Project.