Third-Trimester Ultrasound Fetal Size Predictions in Doubt

October 14, 2020

Late pregnancy scans cannot reliably predict one of the most common delivery complications.

Third-trimester ultrasound is typically used to determine whether a fetus is large for its gestational age, potentially predicting shoulder dystocia, one of the most common complications associated with delivering large babies. But, new research is casting doubt on whether the scan can reliably make this assessment.

Associated with infants who weigh more than 8 lbs. 13 oz. at birth (macrosomia), shoulder dystocia is a major perinatal complication. Ideally, an ultrasound would be able to pinpoint a fetus that will likely have a larger birth weight to help avoid should dystocia. But, in a study published on Oct. 13 in PLOS Medicine, a team of investigators from the University of Cambridge reported that ultrasound fails consistently do so.

“We conclude that ultrasonically suspected [large for gestational age] in the general population has quite good diagnostic effectiveness for macrosomic birth weight,” said the team led by Alexandros Moraitis, M.D., an obstetrics and gynecology trainee at the University of Cambridge. “However, it is not strongly predictive of the risk of associated complications, such as shoulder dystocia.”

Moraitis’s team made the determination after conducting a systematic review of 112,000 pregnancies. Their evaluation showed that, while prenatal ultrasound is highly effective in identifying whether a fetus is large for its gestational age, the scan is only weakly associated with correctly predicting should dystocia. This is particularly true for women who have low- and medium-risk pregnancies.

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In particular, the team examined 41 studies that looked at whether size measurements captured via ultrasound at 24 weeks gestation could be effectively predictive. The team avoided published studies that focused on high-risk pregnancies as they are the focus of another ongoing U.K. trial.

Based on their evaluations, the team discovered that ultrasound-derived size measurements – calculated either with abdominal circumference or estimated fetal weight – can reliably determine which women with low- or medium-risk pregnancies will have a baby with a large birth weight. They also identified a significant relationship between shoulder dystocia and large size measurements identified on ultrasound, but the relationship is weak. Large estimated fetal weight is linked to only a 15-percent greater probability of this complication, they said.

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This finding, they added, underscores the actual incidence of shoulder dystocia.

“The fact that ultrasonic [estimated fetal weight] is relatively poor as a predictor of shoulder dystocia is not unexpected, given that the actual birth weight of the baby is also not strongly predictive of the outcome,” they said. “The majority of cases of shoulder dystocia involve a normal birth weight infant.”

The team also highlighted that simply having ultrasonic evidence of a large baby could lead to complications. Existing studies show that women who erroneously believe – due to ultrasound-derived measurements – that they will deliver a larger infant are more likely to undergo an emergency caesarean section.

“This finding underlines the potential for harm caused by screening low-risk women,” they said.

Moraitis’s team did acknowledge that the quality of the studies included in their review posed a limitation. Many were largely heterogenous and unblinded, and those unblinded studies indicated a stronger link between ultrasound weight predictions and the shoulder dystocia complication. Additionally, only a handful of reviewed studies examined the relationship between size predictions based on ultrasound and newborn morbidity, and they failed to find a link between the prediction of large birthweight and neonatal unit admission.

Overall, the team said, their findings do not support the implementation of universal screening for large birth weight.

“We recommend caution prior to introducing universal third trimester screening for macrosomia, as it would increase the rates of intervention, with potential iatrogenic harm, without clear evidence that it would reduce neonatal morbidity,” they said. “We believe that future studies should address the other factors which help differentiate those suspected [large for gestational age] fetuses which are at the greater risk of complications.”