News release

From: International Congress on Obesity (ICO2026)

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

NOTE – available in Spanish / Comunicado de prensa disponible en español–click here

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.

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.

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.

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.

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.

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.”

ENDS

For more on SERENE see:  https://www.serenestudy.ca/