Study of over 1 million pregnancies shows excessive weight gain during pregnancy increases risk of serious complications in pregnancy regardless of the weight status of pregnant woman
New research presented at this year’s International Congress on Obesity (ICO 2026) hosted by the World Obesity Federation (WOF) in Mexico City, Mexico (15-17 July) shows that, regardless of the weight status of a pregnant woman, excessive weight gain during pregnancy is associated with significant increased risk of serious complications. The study is by Associate Professor Laura Anderson, Department of Health Research Methods, Evidence, and Impact, McMaster Univerisity; and Sahar Khademioore, PhD candidate in the Health Research Methodology Program at McMaster University, Hamilton; ON, Canada and colleagues.
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.
A modifiable factor
Severe maternal morbidity (SMM) comprises potentially life-threatening complications during the perinatal period with long-term physical and mental consequences, such as major hemorrhage or sepsis.
Gestational weight gain is a modifiable factor in pregnancy; however, its association with SMM is not well understood. Therefore, the objective of this study was to examine the association between gestational weight gain and SMM across pre-pregnancy body mass index (BMI) categories.
A retrospective cohort study
The authors did a retrospective cohort study using linked population birth registry data from Better Outcomes Registry & Network (BORN) Ontario and hospitalisation data from Canadian Institute for Health Information (CIHI) spanning April 2012 to March 2022. The cohort included singleton pregnancies with hospital delivery at at least 20 weeks' gestation or birth weight of at least 500 grams. Gestational weight gain was categorised as inadequate, adequate, or excessive per Institute of Medicine (IOM) recommendations. SMM was defined according to Canadian Perinatal Surveillance System criteria. Adjusted risk ratios (aRR) were calculated using statistical modelling.
What the researchers found
Among 1,035,927 pregnancies, 18.6% had inadequate, 28.6% adequate, and 52.8% excessive gestational weight gain; 1.46% (or approximately 3 in every 200) pregnant women experienced SMM. Excessive gestational weight gain, compared to adequate gain, was associated with increased SMM among individuals with all pre-pregnancy BMIs including underweight (33% increased risk), normal weight (24%), overweight (16%), class I obesity (21%), class II obesity (28%), and class III obesity (24%).
In contrast, inadequate gestational weight gain was associated with increased SMM predominately in those with normal weight (13% increased risk) and overweight (19%) BMI categories. The authors observed that lower weight gain was a strong risk factor for SMM among women who were underweight, normal weight or overweight prepregnancy but less so among women living with obesity.
When risks combine
Pre-pregnancy medical conditions and gestational weight gain outside of recommended range showed synergistic additive interactions. For example, excessive gestational weight gain in people with diabetes or hypertension was associated with disproportionately higher risk of SMM than would be expected by each of these risk factors alone. For example, a normal-weight woman with a pre-existing condition such as chronic hypertension or diabetes who also gained excessive weight had close to three times the risk of severe maternal morbidity compared with a normal-weight woman with adequate weight gain and no such condition, a risk greater than would be expected from simply adding the two factors together.
Informing future guidelines
The authors conclude: “Gestational weight gain outside recommended ranges may be associated with increased severe complications in the pregnant women, with effect patterns varying by pre-pregnancy BMI. These findings may inform future refinement of gestational weight gain guidelines.”
Ms Khademioore adds:
“Healthy pregnancy weight gain isn't just important for women with a higher BMI, it matters for all pregnant women whatever their weight. We found that excessive weight gain was common and increased the risk of serious complications across all BMI categories. This highlights the need for more resources to support healthy pregnancy weight gain and improve outcomes for mothers and babies.”
Associate Professor Laura Anderson, Department of Health Research Methods, Evidence, and Impact, McMaster Univeristy, Hamilton, ON, Canada. Please e-mail with questions and interview requests. E) ln.anderson@mcmaster.ca
Tony Kirby in the ICO Media Centre. T) +44 7834 385827 (WhatsApp) E) tony.kirby@tonykirby.com
The authors declare no conflicts of interest.
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.
For more on SERENE, please click here.
This press release is based on a poster abstract at the International Congress on Obesity (ICO) in Mexico City. All accepted abstracts have been extensively peer reviewed by the congress selection committee. There is no full paper at this stage, but the authors are happy to answer your questions. The research has been submitted to a medical journal for publication.
The press release is also available in Spanish, please click here.
Read the full abstract
The full abstract 'Weight gain during pregnancy and severe maternal morbidity in Ontario,
Canada: a population-based cohort study' is available via the link below.