New research shows fetal weight at birth impacts delivery for mothers with gestational diabetes

by Barry Waldman

For high-risk births where the risk of stillbirth increases with fetal age but the risk of other complications rise with early delivery and low birth weight, how should obstetricians balance the confounding variables? A new study led by an MUSC researcher will help clinicians calculate that very risk-reward equation.

The study considered the babies of mothers with Type 1/Type 2 pregestational diabetes and gestational diabetes – that is, diabetes whose onset is spurred by pregnancy and usually resolves itself following delivery. Stillbirth is defined by the CDC as intrauterine death beyond 20 weeks of gestation and occurs in approximately one in 160 deliveries in the U.S. Diabetes dramatically increases that risk, and in a state in which one in seven individuals has diabetes, this is a significant issue for obstetricians and gynecologists.

The risk of stillbirth in patients with diabetes is four to five times that in the general population, according to previous studies, and diabetes is more than twice as prevalent among Black women as white, according to the Department of Health and Environmental Conservation. Other co-factors with diabetes include smoking, high blood pressure, high cholesterol, obesity and a sedentary lifestyle. It was the eighth leading cause of death in the Palmetto state in 2020.

Research led by Eliza McElwee, M.D., an OB-GYN trained at MUSC and practicing at MUSC Health as well as an assistant professor at the College of Medicine, studied the variable impacts on the risk of stillbirth of the two forms of maternal diabetes. She focused on the size and age of the fetus at birth or at stillbirth for her study which was published in the journal Obstetrics and Gynecology.

Diabetes can cause fetuses to lag in development, or paradoxically, to grow unusually large as glucose crosses the placental barrier and stimulates fetal growth. Fetuses of diabetic women growing abnormally large are often a sign that the diabetes is difficult to control. Both under-developing and over-developing fetuses increase the risk of stillbirth even further.

Working with data from more than 800,000 pregnancies complicated by diabetes, McElwee and her team teased out the risks of these three variables: type of diabetes, gestational age of the fetus and the size of the fetus.

“We found that pregnancies complicated by pregestational diabetes (Type 1 or Type 2) have a higher risk of stillbirth compared to women with gestational diabetes,” McElwee said. “The greatest risk of stillbirth was in women with pregestational diabetes and large babies. These patients have a stillbirth risk 21 times that of women with normal size babies and gestational diabetes.”

Moreover, the research found that those risks increase in mothers with diabetes as doctors wait to deliver the baby beyond the 34th week of pregnancy, particularly with large gestational size. For mothers with pre-gestational diabetes, large babies allowed to develop to term suffer a stillbirth rate seven times those delivered after 34 weeks.

For small fetuses, the effect of delivery date is more varied and less dramatic, with full-term pregnancies a bit more than twice as likely to result in stillborn babies. These impacts are similar but at much lower levels when the fetus is abnormally small, the research found.

McElwee points out that while it would seem that delivering earlier is indicated, particularly in those cases where a diabetic mother is carrying a large fetus, this conclusion is confounded by another variable: while the risk of stillbirth increases as the fetus ages, the risk of other complications, including post-partum death of the baby, increases when they are delivered too early. Babies delivered prematurely at 34 weeks of gestation have a 40-fold incidence of fatal respiratory issues compared to those delivered at term, and that risk declines with each additional week of gestation.

Consequently, delivery timing in pregnancies with diabetes remains a clinical challenge with a complex matrix of considerations, McElwee says.

“We must weigh the risk of further pregnancy complication – such as maternal health worsening or fetal stillbirth risk – versus the potential complications of prematurity such as problems with brain and lung development or even death,” she said.

The use of medication and the ability to control the mother’s insulin levels are also variables to consider when deciding delivery timing for mothers with diabetes, she said.

This research is particularly urgent because previous studies have found that as the reproductive age of women increases, so does their incidence of diabetes. A recent study found that roughly 13% of women are diabetic, but more than a quarter of them are undiagnosed and more than half the cases of diabetes go uncontrolled. These present potentially dangerous complications during pregnancy.

Clinicians have long considered variables such as maternal diabetes and medication when determining birth timing, but not abnormal fetal growth. Fetal growth is already assessed in cases of maternal diabetes, and McElwee’s research suggests it should be included among the variables considered.

“Our research suggests that the size of the fetus is another factor that should be taken into consideration when coordinating delivery timing for patients with diabetes,” she said. But more research is needed to assess the complex decision-making that can aid in delivery timing recommendations, including glycemic control and type of diabetes.