Maternal obesity and obesity with other risk factors are associated with an increased risk of stillbirth

By Dr. Liji Thomas, MDReviewed by Danielle Ellis, B.Sc.Mar 5 2024

Prior research indicates an increase of two- to five-fold in stillbirth risk among women with obesity. Not enough is known about how severe degrees of obesity impact live birth outcomes.

A recent study in CMAJ compared the risk of stillbirth among obese mothers in two classes. One had only obesity as a risk factor, whereas the other had undiagnosed or additional high-risk factors affecting pregnancy.

Introduction

Factors like pre-existing diabetes or hypertension interact with obesity to increase pregnancy risk. Such conditions are also more likely to occur among obese mothers. Pregnancy complications like gestational hypertension and gestational diabetes, or fetal growth restriction, are also more common among obese women and are associated with raised stillbirth rates.

A better understanding of how these work together is necessary to offer proper advice to women with such risk factors, both with regard to early pregnancy care and the best timing of delivery.

The data came from the Better Outcomes Registry and Network (BORN), Ontario, and included only singleton hospital births. The study period was between 2012 and 2018. The aim was to assess how the body mass index (BMI) affected stillbirth risk, directly and indirectly, after compensating for other contributing factors and independent risk factors.

What did the study show?

The study included approximately 680,000 births. There were nearly 2,000 stillbirths among them.

Stillbirths were more likely among those with obesity. However, they were also more common among those who had no prior childbirths, those who used assisted reproduction technologies, and those with lower family income. In addition, those who used substances or smoked, as well as those with pre-existing medical conditions, including hypertension and diabetes, were at higher risk.

Obesity was more common among women with more children, lower financial status, and those who smoked. Both class I and II obesity were more strongly associated with stillbirths occurring prior to delivery vs women with normal BMI. Class II and II obesity were more likely to have stillborn babies, along with gestational hypertension and diabetes.

Stillbirths were also more likely to occur along with congenital malformations and growth-restricted fetuses compared to the whole pregnant population in the country.

Overall, there was a strong increase in stillbirth risk with obesity after compensating for some or all confounding factors identified in this study. The risk also increased with gestational age.

Class I obesity was linked to 56% higher odds of having a stillbirth after 37 complete weeks of gestation compared to those with normal BMI. The strongest association was with class II obesity, however, where the odds were more than doubled, and class III, with an 80% increase in the odds.

Thus, at term, obesity with or without other risk factors was associated with a significant increase in the odds of stillbirth at term or beyond, compared to pregnancies in non-obese women. The risk increases with gestational age and is maximum at term (37 completed weeks of gestation).

For class I obesity, the increased risk was seen only at 39 weeks, when it was twice as high as among normal-BMI women. In class II and III obesity, the risk was higher at 38 and 40 weeks than among normal-BMI women. Class II obesity showed a further peak at 41 weeks.

Among women with class II or III obesity, even higher risks were observed at 38 weeks, at 3.5 and 2.6 times that among normal-BMI women. The stillbirth rate rose still further at 40 weeks, however. The lower risks at 37 and 39 weeks could be because of smaller sample sizes at these ages.

The risk was doubled at 38 weeks for women with pre-existing hypertension or diabetes but dropped thereafter. This could be because standard recommendations ensure that most women with such conditions are delivered at this gestational age.

Only about 120% and 1% of the risk was explained by preterm births and hypertensive disorders in the obesity cohort after accounting for all potential confounders. This does not indicate a definitive role of these conditions in mediating the association of obesity with stillbirth. Conversely, small-for-gestational-age (SGA) infants were less likely to be stillborn, perhaps because they were monitored more carefully.

The study supports prior observations that obesity, especially in higher classes, puts women at higher risk for stillbirth, increasing with gestational age. This is so even after allowing for other possibly undiagnosed factors.

What are the implications?

Previous studies using the same data suggested that the risk of pre-eclampsia, as well as other maternal and fetal complications around the time of delivery, could be reduced by delivery at this gestational age, supporting these recommendations. In fact, in 2014, another study suggested that 200 stillbirths would be averted by delivering babies at 38 weeks for women with obesity, compared to waiting until 41 weeks. This corroborates the above conclusion for higher classes of obesity.

“Our findings suggest that delivery around 39 weeks’ gestation for pregnant people with class I obesity and 38 weeks’ gestation for pregnant people with class II or III obesity may help mitigate the risk of stillbirth.”