Study: As BMI increases, so does the risk for poor pregnancy outcomes

Key takeaways:

• Women with a BMI of 40kg/m² or more had the greatest risk for maternal morbidity, including preeclampsia.
• The findings could help inform antenatal counseling in pregnant women, researchers said.

A new study in the Journal of Osteopathic Medicine showed that higher BMIs before pregnancy correlated with increasing risks for maternal and neonatal morbidity.

“The obesity epidemic continues to rise in the U.S. and obesity is associated with adverse pregnancy outcomes,” Awathif Dhanya Mackeen, MPH, MD, director of research for the division of maternal-fetal medicine at Geisinger Medical Center, told Healio. “We wanted to determine whether the class of obesity affected the likelihood of having adverse pregnancy or neonatal outcomes so that patients could be counseled accordingly, preferably preconception.”

Mackeen and colleagues conducted a retrospective cohort study to evaluate the effects of BMI on maternal and neonatal morbidity in 40,256 pregnant women with 55,202 singleton births between Oct. 16, 2007, and Dec. 3, 2023.

The researchers categorized BMI before pregnancy as:

• normal weight (18.5 to 24.9kg/m²);
• overweight (25 to 29.9kg/m²);
• class I obesity (30 to 34.9kg/m²);
• class II obesity (35 to 39.9kg/m²); and
• class III obesity (40kg/m² and higher).

Overall, 26.1% of women had overweight, 16.8% had class I obesity, 10.3% had class II obesity and 8.6% had class III obesity.

The study’s primary outcome was composite maternal morbidity, which included hypertensive disorders of pregnancy and gestational diabetes mellitus. Secondary outcomes included preeclampsia, cesarean delivery, induction of labor and postpartum hemorrhage.

Compared with women who had normal weight, the ORs for composite maternal morbidity were:

• 1.72 (95% CI, 1.63-1.82) for those with overweight;
• 2.66 (95% CI, 2.5-2.84) for those with class I obesity;
• 3.35 (95% CI, 3.08-3.65) for those with class II obesity; and
• 4.4 (95% CI, 3.7-5.22) for those with class III obesity

“An increased risk was also seen for composite neonatal morbidity, although the increase was not as strong as maternal morbidity,” the researchers noted.

The odds for several other neonatal and maternal risks also rose with increasing BMI classes.

For example, those with class III obesity were 2.35 (95% CI, 1.74-3.19), 4.55 (95% CI, 3.67-5.63) and 3.07 (95% CI, 2.08-4.54) times likelier to develop preeclampsia, gestational hypertension and postpartum hemorrhage, respectively, compared with those with normal pregnancy BMI.

Additionally, there was a greater risk for a preterm delivery less than 32 weeks and increased odds for macrosomia among obesity classes vs. women with normal weight.

“The increase in macrosomia is clinically relevant, as those with class II obesity had 1.63 increased odds of having a macrosomic neonate as a patient with normal weight,” Mackeen and colleagues wrote.

The study cohort was mostly non-Hispanic white women, which the researchers noted may hurt the findings’ generalizability to other populations. Additionally, some adverse outcomes that may occur more often in diverse populations — like preterm birth or hemorrhage — may not have occurred as much in the study population.

“Our hope is that this data will encourage providers to counsel and motivate patients to attain a healthier BMI prior to conception to decrease their risks of pregnancy morbidity,” Mackeen said. “Next steps include determining how changes in BMI during pregnancy should affect management of the pregnancy since most of the management recommendations are based on pre-pregnancy BMI.” 

For more information:

Awathif Dhanya Mackeen, MPH, MD can be reached at admackeen@geisinger.edu.