Led by researchers from McMaster University, an analysis of over one million births discovered that putting on too much weight throughout pregnancy is connected to an elevated danger of life-threatening maternal complications, regardless of the individual’s initial weight.
McMaster University Researchers Analyze Over One Million Births
Published on Oct. 22, 2026, in JAMA Network Open, the research evaluated more than one million Ontario pregnancies using the BORN Ontario birth registry. The findings challenge current guidelines, which were developed primarily to improve outcomes for babies, such as ensuring a healthy birth weight and reducing the risk of preterm birth.
The study suggests these existing recommendations should also be evaluated through the lens of maternal health. Researchers identified more than 15,000 severe maternal morbidity cases within the dataset.
Excessive Weight Gain Increases Risk Across Every BMI Category
Data from the registry revealed that 53 per cent of pregnancies involved weight gain above current recommendations, while 19 per cent gained less than recommended. Excessive weight gain increased the risk of severe complications across every body mass index (BMI) category.
“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,” said 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.”
Pre-Existing Health Conditions Double the Risk of Severe Maternal Morbidity
Individuals who had prior medical conditions like high blood pressure, asthma, and diabetes experienced roughly twice the hazard of severe maternal complications. These risks increased further when weight gain fell outside recommended ranges. The analysis showed that gaining less than the recommended amount of weight was associated with higher risks of severe maternal outcomes in certain 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,” said Laura Anderson, senior author and associate professor with McMaster’s Department of Health Research Methods, Evidence, and Impact. “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.”
Funding and Research Context Behind the SERENE Project
This research operates as part of the SERENE Project. The initiative runs in partnership with a lived experience advisory network committed to exploring why and when severe maternal morbidity happens, which people are most affected, and the long-term physical, social, and mental health implications involved.
Funding for the study was provided by the Juravinski Research Institute, a partnership established through the generosity of the late Margaret and Charles Juravinski involving Hamilton Health Sciences, McMaster University, and St. Joseph’s Healthcare Hamilton.
Frequently Asked Questions About the McMaster Pregnancy Study
What specific complications were linked to excessive pregnancy weight gain?
The study associated excessive weight gain with life-threatening maternal complications during childbirth, specifically including bleeding, pre-eclampsia, and ICU admission.
What data source did the researchers use for the analysis?
Researchers utilized the BORN Ontario birth registry to analyze more than one million Ontario pregnancies.
When was the study published?
The study was published on Oct. 22, 2026, in JAMA Network Open.
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