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Published in CenterView on April 29, 2013
John Morrison
John Morrison

Study helps ob-gyns differentiate BMI thresholds for at-risk mothers-to-be

By Jack Mazurak

The growing obesity crisis means pregnant mothers will more often run up against increasingly severe complications, leaving doctors, especially in rural areas, to transfer women to tertiary-care centers like the University of Mississippi Medical Center.

But knowing which women face risk for which problems could help doctors intervene earlier, cutting expense and suffering and keeping more babies alive and moms healthy.

In a new study, researchers used data from nearly 4,500 pregnancies in Mississippi and Virginia to sort out body mass index (BMI) thresholds for gestational diabetes, preterm labor, endometritis, preeclampsia and other complications.

Obesity among U.S. women increased from 33.4 percent in 1999-2000 to 35.8 percent a decade later, according to the Centers for Disease Control and Prevention. Nationwide, 69.2 percent of adults were overweight or obese in 2010.

Obstetricians have known for a long time that more weight means worse complications, said study co-author Dr. John Morrison, professor and chair emeritus of obstetrics and gynecology.

“But we were being asked, ‘When does one condition develop versus another?’” he said. “What doctors want to know is, at what point do you look for one thing versus another? When should you refer a patient to a tertiary-care hospital rather than taking on a dangerous situation yourself?”

Morrison worked with Dr. Pat Magann, professor and director of maternal-fetal medicine at the University of Arkansas for Medical Sciences, and researchers at the University of Western Australia.

In 2007 and 2008, they tracked 4,490 women through pregnancies, recruiting mothers at UMMC and the Naval Medical Center in Portsmouth, Va. Using that data, the researchers retrospectively grouped women into seven BMI categories and analyzed the complications each woman faced.

With that analysis, they identified a threshold for each complication.

For example, only 2.8 percent of underweight and normal-weight mothers – classified as BMI between 18.5 and 25 – developed gestational diabetes.

But that jumped to 6 percent in overweight mothers (BMI 25-29.9), the threshold for the particular complication. It increased to 9 percent among obese mothers BMI 30-34.9, further to 11.4 percent in mothers with BMIs 35 to 39.9, and topped out at 15 percent in mothers with BMIs 40-44.9.

Preeclampsia, which is dangerously high blood pressure and protein in the urine, becomes significantly more likely in above 30 BMI. The rate doubled from 6.1 percent in overweight to 12 percent in obese women. It continued to 14.2 percent in women BMI 35 - 39.9, jumped again to 23.4 percent in women BMI 40-44.9 and up to 29.6 percent in women with BMIs 45 and greater.

The Australian and New Zealand Journal of Obstetrics and Gynecology published the study online in February. The authors want their findings put into practice.

“For example, urinary tract infection kicks in with patients with BMIs 35 and above,” Morrison said. “We’d say start doing urine cultures – which is not normal – on those patients. That screening would catch UTIs early and reduce complications.”

Some complications snowball. A mother’s gestational diabetes ups the chance her baby will be large for its age. That increases the likelihood of shoulder dystocia, an emergency during delivery when one of the baby’s shoulders can’t pass through the birth canal.

“If we could just pick up diabetes early, we could reduce the risk of large babies by putting the patients on diet and insulin,” he said.

That could reduce Caesarean deliveries, which decreases infection risks from surgical wounds and in the uterine lining. Reducing the baby’s size by treating gestational diabetes early lessens the chance the child will have metabolic syndrome.

The authors noted their analysis found different thresholds for different complications.

“Our study suggests that as the pre-pregnancy BMI gradient increases, the patient becomes susceptible to additional pregnancy complications, thus confirming that all obese BMI categories do not carry the same pregnancy risks,” the authors wrote.

“You have to remember, the complications don’t go away in overweight and obese women. They continue as other complications present themselves,” Morrison said.

Applied in practices, the threshold findings could save lives.

“We’d like to take this data and put it to work in our clinics,” Morrison said. “Ideally, we’d like to limit Caesareans and make earlier diagnoses of gestational diabetes. We want to address the complications and treat them effectively.”