WASHINGTON – , John C. Hobbins, MD, said at the biennial meeting of the Diabetes in Pregnancy Study Group of North America.
“Everything that the estimated fetal weight [EFW] can do, the abdominal circumference can do better,” especially in diabetic mothers, said Dr. Hobbins, who is widely regarded as one of the early pioneers in the development and use of obstetric ultrasound as a diagnostic tool.
Because it reflects liver size and incorporates subcutaneous fat, the abdominal circumference (AC) “concentrates on where the action is,” he said. It also “roughly correlates” with the size of the fetal shoulders and is not affected by genetic factors.
Moreover, “it focuses on one task rather than putting into play four variables, each with its own standard error of the method,” said Dr. Hobbins, referring to the four fetal biometric parameters incorporated into the Hadlock formula for EFW that is provided “upon fire-up of virtually every ultrasound machine.”
These four parameters (biparietal diameter, head circumference, femur length, and AC) each contribute to fetal weight but the AC has been shown to correlate better with weight at birth than the other variables, and it is the only measure that reflects how corpulent the fetus is, he noted.
The “general rule of thumb is that the EFW [as calculated by ultrasound machine–based equations] has a standard error of the method of plus or minus 10%. … which means that an EFW of 4,000 g is associated with a splay of plus or minus 1.2 pounds,” said Dr. Hobbins, professor of obstetrics and gynecology at the University of Colorado at Denver, Aurora.
“But the problem for us is not the failure of the ultrasound – it’s the way we use it,” he said.
An assortment of customized formulas have been developed for macrosomia, including one designed for use in diabetes that incorporates AC, head circumference, femur length, and 3D volumes of the thigh and abdomen. “If one used this and set a cut-off at 4,300 g, you’d pick up 93%. … but at false positive rate of 38%,” he said.
While not perfect, the AC alone is as accurate as more complicated formulas to detect macrosomic fetuses, he emphasized. “It’s a tough [measurement] to get just before the baby is born, but you can do it a little bit earlier,” he said. Research has shown that screening at 30-34 weeks can capture a majority of the fetuses destined to be greater than 4,000 g at birth, with a lower false-positive rate.
He pointed to one “very interesting” recent study in which AC was measured with a handheld ultrasound device at 24-40 weeks’ gestation, prior to formal ultrasound estimation of EFW. Early AC was a better predictor of large-for-gestational age babies at birth than EFW or early fundal height measurement, with sensitivities of 67%, 25%, and 50%, respectively (Am J Obstet Gynecol. 2015 Jun;212[6]:820.e1-8).
“And AC had a false-positive rate of only 10%,” Dr. Hobbins said.
Macrosomia occurs in 20% of cases of gestational diabetes and 25% of pregestational diabetes, he said. “And even if there is adequate glucose control, 17% will be macrosomic.”
The condition correlates with childhood and adulthood metabolic dysfunction and is associated with significantly increased risk of birth injury to the infant and to the mother. The alternative – cesarean delivery – is “not innocuous,” and “[we have] a very low threshold for cesarean if macrosomia is suspected,” Dr. Hobbins said.
Dr. Hobbins reported having no financial disclosures.