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A new cohort study has found an increased risk for stillbirth among women with obesity, particularly at term. 

The study analyzed data from > 680,000 births in Ontario to evaluate this risk. By using a larger database than in previous studies, the investigators were able to control for potential confounders, including comorbidities, and stratify the results by gestational age and obesity class. While the risk was elevated for all classes, the strongest association was found with class II obesity (adjusted hazard ratio, 2.17), and the risk increased significantly after 37 weeks' gestation.

"The study identified a population that is at risk of stillbirth and that risk level at term is very high" relative to other populations, study author Naila Ramji, MD, a maternal-fetal medicine specialist and assistant professor of obstetrics and gynecology at Dalhousie University in Halifax, Canada, told Medscape Medical News. These findings may indicate the benefit of delivering earlier in patients with obesity, regardless of comorbidities.

The study was published on March 4, 2024, in the Canadian Medical Association Journal.

Increased Risk

While patients with a higher BMI are known to be at an increased risk for stillbirth, the outcome is rare. To study the association, the investigators drew data from the Better Outcomes Registry and Network, a validated database that records births of > 20 weeks' gestation in Ontario, Canada. The data included 681,178 births, of which 1956 were stillbirths.

With this large dataset, the investigators were able to control for several potential confounders associated with stillbirth, including maternal age > 35 years, smoking, substance use, and pre-existing health conditions such as diabetes and hypertension. The association between obesity and increased incidence of stillbirth persisted when controlling for these confounders.

The investigators also explored the role of gestational age. "When we looked at the relationship over time, we could see that the risk goes up significantly at term," said Ramji. At 38 weeks' gestation, the risk for stillbirth among those with class II obesity is 3.0-3.5 times higher than those with normal BMI (18.5-24.9). That risk rises to 4.0-4.5 times higher by 40 weeks. 

In addition, the study separated the data into the three obesity classes. "A lot of studies just look at obesity: Yes or no," said Ramji. But when they separated patients into obesity classes, the investigators found the strongest association with increased incidence of stillbirth in class II obesity, which is defined as a BMI between 35.0 and 39.9. The investigators suggest that the relatively lower risk among those with class III obesity may be explained by providers anticipating potential complications in this population and delivering earlier.

Bias and Stigma

In an editorial accompanying the study, Naomi Cahill, PhD, a registered dietitian and nutrition researcher, discussed the importance of addressing the stigma that patients with obesity face when caring for pregnant patients. "Focusing on weight during communications of risk may reinforce weight bias, weight stigma, and discrimination for pregnant people," wrote Cahill. She called for weight-inclusive practices such as providing appropriately sized equipment and discussing symptoms of overall health, rather than BMI, to be prioritized. 

Ramji agreed that it is important to consider how providers communicate the risks that come with obesity during pregnancy but added that those risks should not be brushed off. "Our concern is that the health risks that are related to maternal obesity and pregnancy are potentially not being taken seriously enough, when you look at how women with other medical conditions are treated." While patients with diabetes or chronic hypertension are often delivered before their due dates, this is not standard for patients with obesity. "For maternal obesity, there seems to be a higher level of risk tolerance," Ramji said.

These results should be considered in guidelines for the timing of delivery, said Ramji. "I think that there is enough evidence to be making recommendations for delivery around 38-39 weeks in women with obesity." 

'Long-Standing Question'

Commenting on the study for Medscape Medical News, Kristin Harris, MD, a gynecologist and assistant professor of maternal-fetal medicine at the University of Toronto in Ontario, said that the study provided a well conducted analysis of the risks associated with obesity and comorbidities during pregnancy. Deciphering where the risk lies, whether with obesity itself or associated conditions, has been "a long-standing question," said Harris. 

She added that the findings regarding gestational age should be considered when counseling patients on the timing of their deliveries and on whether to induce or continue the pregnancy to term. 

In response to Cahill's editorial, Harris said that it's important to acknowledge the weight bias to "build a better foundation for trust and care," noting that previous studies have found that stigma may affect the quality of care, particularly for those with a BMI > 40. 

"There are a lot of opportunities to explore how we can better serve this patient population," said Harris. That service may include further evaluating delivery planning, including methods and timing of induction, as well as better understanding the stigma and bias patients may face. 

The study was supported by the Canadian Institutes of Health Research and the Dalhousie University Advancement Funding Grant. Ramji and Harris reported no relevant financial relationships.

Gwendolyn Rak is a health reporter for Medscape based in Brooklyn, New York.