Scientists studied 1 million pregnancies, then found one overlooked risk

Gaining weight during pregnancy is inevitable, but gaining too much may raise the risk of potentially life-threatening complications—regardless of a woman’s starting weight.

That’s according to a new study of more than 1 million pregnancies, which found that excessive weight gain during pregnancy was associated with a higher risk of severe maternal morbidity (SMM)—a group of serious complications that can include major hemorrhage and sepsis.

The study, led by associate professor Laura Anderson of McMaster University’s Department of Health Research Methods in Ontario, Canada, found that the increased risk ranged from 16 percent to 33 percent across all body mass index (BMI) categories, including women who started pregnancy underweight, at a healthy weight, overweight or obese.

The greatest increase was seen among women who were underweight before pregnancy, who faced a 33 percent higher risk of severe complications compared with those who gained the recommended amount of weight.

“Healthy pregnancy weight gain isn’t just important for women with a higher BMI; it matters for all pregnant women whatever their weight,” Sahar Khademioore, a Ph.D. candidate in the Health Research Methodology Program, said in a statement.

“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.”

Newsweek reached out to the researchers for a comment. We are yet to hear back.

Serious Risks Linked to Excess Weight Gain

The researchers examined the relationship between pregnancy weight gain and severe maternal morbidity (SMM).

SMM refers to potentially life-threatening complications that occur during pregnancy, childbirth or shortly afterward. These conditions can have long-term physical and mental health consequences for mothers.

The recommended amount of weight gain during pregnancy depends on a woman’s pre-pregnancy BMI. For a singleton pregnancy, the Institute of Medicine (IOM) recommends gaining 28-40lbs for women who are underweight, 25-35lbs for those at a healthy weight, 15-25lbs for those who are overweight, and 11-20lbs for those with obesity.

To investigate the link, researchers analyzed population birth registry data and hospitalization records in Ontario, Canada, covering the period from April 2012 to March 2022.

The study included 1,035,927 singleton pregnancies delivered in hospital.

Overall, 52.8 percent of women gained more weight than recommended during pregnancy, while 28.6 percent gained an appropriate amount and 18.6 percent gained less than recommended. Roughly 1.46 percent of women—about three in every 200 pregnancies—experienced severe maternal morbidity.

Compared with women who gained the recommended amount of weight, excessive pregnancy weight gain was associated with a higher risk of SMM across every BMI category:

• Underweight: 33 percent increased risk
• Normal weight: 24 percent increased risk
• Overweight: 16 percent increased risk
• Class I obesity: 21 percent increased risk
• Class II obesity: 28 percent increased risk
• Class III obesity: 24 percent increased risk

Existing Health Conditions May Add to the Risk

The researchers also found that pre-pregnancy medical conditions such as diabetes and hypertension appeared to amplify the risks associated with gaining too much weight.

For example, a woman who started pregnancy at a normal weight but also had chronic hypertension or diabetes and gained excessive weight during pregnancy faced nearly three times the risk of severe maternal morbidity compared with a normal-weight woman with no preexisting condition who gained an appropriate amount of weight.

The study also found that gaining too little weight was linked to increased risk in some groups, particularly women who began pregnancy at a normal weight or overweight.

Weight Gain Matters at Every Size

The researchers said their findings show that healthy pregnancy weight gain is important for all women, not just those who begin pregnancy overweight or obese.

The authors concluded that pregnancy weight gain outside recommended ranges may be associated with an increased risk of severe complications, with patterns differing according to pre-pregnancy BMI, and said the findings could help inform future updates to pregnancy weight-gain guidelines.

Reference

McMaster University (2026). Excessive weight gain during pregnancy increases risk of serious complications, no matter your weight. [online] Medical Xpress. Available at: https://medicalxpress.com/news/2026-07-excessive-weight-gain-pregnancy-complications.html

Contact Newsweek editors on this story: Kara Dolman and Emma Lee-Sang