Fetal ultrasound technique does not improve prediction of small-for-gestational age babies

USF Health’s study finds no added benefit of the cerebroplacental ratio approach over standard umbilical artery Doppler ultrasound in evaluating pregnancies complicated by fetal growth restriction

Fetal growth restriction, also known as small for gestational age, or SGA, remains a leading contributor worldwide to the death and illness of babies before and after birth. The condition is commonly defined as ultrasound-estimated fetal weight below the 10th percentile for gestational age; the unborn baby is smaller than expected for the number of weeks of pregnancy (gestation).

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In the United States, umbilical artery (UA) Doppler ultrasound is the standardized tool used to determine risk for SGA that can lead to adverse outcomes such as stillbirth, hypoxia and respiratory distress. However, some retrospective studies have suggested that cerebroplacental ratio (CPR), which supplements UA ultrasound with middle cerebral artery Doppler ultrasound, may be superior to UA alone in identifying adverse pregnancy outcomes linked to poor fetal growth.

So, researchers at the University of South Florida Health (USF Health) Morsani College of Medicine, Tampa, Fla., conducted a prospective study comparing the effectiveness of CPR to UA Doppler ultrasound alone in predicting SGA and short-term adverse outcomes in newborns. Their findings were published online July 21 in The Journal of Maternal-Fetal & Neonatal Medicine.

“We asked whether cerebroplacental ratio is a technology that can improve our evaluation of physiologically small babies, and overall our study found that it did not,” said principal investigator Anthony Odibo, MD, professor in the USF Health Department of Obstetrics and Gynecology and director of the Ultrasound and Fetal Therapy Program.

A normal umbilical artery Doppler ultrasound waveform, with the reddish color representing the umbilical artery and blue the vein

The ability of the placenta to shuttle blood, oxygen and nutrients from mother to the growing fetus through an intricate network of blood vessels is inadequate in pregnancies complicated by fetal growth restriction.

UA Doppler ultrasound measures blood flow from the placenta through the umbilical cord to the fetus, while middle cerebral artery (MCA) Doppler ultrasound assesses blood circulation in the fetal brain.  CPR combines both these imaging techniques: UA/MCA = CPR.  Basically, Dr. Odibo said, the ratio accounts for what happens when the fetus can no longer adapt to placental insufficiency by redistributing blood that carries oxygen and nutrients to vital organs like the brain, known as the “brain sparing” effect.

The USF study reported that abnormal UA was more sensitive than low CPR in accurately predicting SGA (which babies would weigh less than the 10th percentile when born) and in identifying poor neonatal health outcomes.  However, CPR performed the same as UA in test specificity, that is, in correctly ruling out SGA and adverse neonatal outcomes.

Ultrasound image of a normal fetus at 20 weeks gestational age

The researchers recruited more than 1,000 pregnant women at high risk for fetal growth restriction, including maternal weight less than 100 pounds, smokers and those with placental abnormalities, diabetes and chronic high blood pressure. The women were referred for specialized ultrasounds to USF Health Morsani College of Medicine and Washington University School of Medicine, St. Louis. Both UA and MCA Doppler ultrasounds were ultimately performed on 199 women, between 26 and 36 weeks of pregnancy, who met the study criteria. Two groups of demographically similar study participants were compared – one with abnormal UA ultrasounds and normal CPRs and another with abnormal (low) CPRs. Because the sample size was relatively small, the researchers defined adverse health outcomes in the newborns as one or more of the following assessments:  low pH for umbilical artery blood flow, an abnormal Apgar score 5 minutes after delivery, admission to the neonatal intensive care unit, respiratory distress syndrome, and/or low blood sugar.

While this particular study showed no added benefit in using CPR compared to UA ultrasound alone, the study authors emphasized that larger standardized clinical studies are warranted to definitively determine the effectiveness of CPR in routine evaluation and monitoring of pregnancies complicated by fetal growth restriction.

Anthony Odibo, MD, is professor of obstetrics and gynecology and director of the Ultrasound and Fetal Therapy Program at USF Health.

“Negative studies are equally important in advancing evidence based medicine,” Dr. Odibo said. “Pooling both positive and negative findings can more conclusively determine whether a new technology is useful to introduce into clinical practice or not.”

The study was supported by a Human Placenta Project grant from the Eunice Kennedy Shriver National Institute of Child Health and Human Development.