Third trimester ultrasound may detect macrosomia, not shoulder dystocia: Study

Researchers have found in a new study that the benefits of using third-trimester ultrasound scans to predict large infant size may be limited. Ultrasound size estimates failed to reliably predict one of the most common complications of delivering large newborns i.e shoulder dystocia.

Third-trimester fetal ultrasound screening can identify more pregnancies with macrosomia (weighing more than 8 pounds, 13 ounces at birth), but failed to detect.

The study is published in PLOS Medicine.

Delivering a newborn with macrosomia may be associated with a higher risk of adverse outcomes, including perinatal death and injuries related to traumatic delivery, such as stuck shoulders (shoulder dystocia).

The diagnostic effectiveness of ultrasound screening in predicting the delivery of a macrosomic infant, shoulder dystocia and associated neonatal morbidity is not well established. To better understand the relationship between estimated fetal weight (EFW), macrosomia, and perinatal complications, researchers systematically reviewed the literature from four different clinical databases.

One of the critical elements of an effective screening program is the diagnostic accuracy of a test at predicting the condition.

"We conducted a predefined literature search in Medline, Excerpta Medica database (EMBASE), the Cochrane library and ClinicalTrials.gov from inception to May 2020. We included studies where the ultrasound was performed as part of universal screening and those that included low- and mixed-risk pregnancies and excluded studies confined to high risk pregnancies", describes Alexandros A. Moraitis, the lead author of the study.

"We used the estimated fetal weight (EFW) (multiple formulas and thresholds) and the abdominal circumference (AC) to define suspected large for gestational age (LGA). Adverse perinatal outcomes included macrosomia (multiple thresholds), shoulder dystocia, and other markers of neonatal morbidity", he further added.

The authors then analyzed 41 studies that met the inclusion criteria involving 112,034 non-high-risk patients who had undergone a third-trimester ultrasound screening as part of universal screening.

The following findings were highlighted-

a. Both EFW >4,000 g (or 90th centile for the gestational age) and AC >36 cm (or 90th centile) had >50% sensitivity for predicting macrosomia (birth weight above 4,000 g or 90th centile) at birth with positive likelihood ratios (LRs) of 8.74 and 7.56, respectively.

b. There was significant heterogeneity at predicting macrosomia.

c. An EFW >4,000 g (or 90th centile) had 22% sensitivity at predicting shoulder dystocia with a positive likelihood ratio of 2.12.

d. There was insufficient data to analyze other markers of neonatal morbidity.

The authors found that a third trimester ultrasonic EFW showing an increased risk of a large baby reliably predicted delivery of a macrosomic infant. However, a larger EFW was not strongly associated with the risk of shoulder dystocia in low and medium-risk pregnancies. The study was limited by variation in included studies representing differences in screening in various countries.

According to the authors, "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."

For further reference log on to:

Moraitis AA, Shreeve N, Sovio U, Brocklehurst P, Heazell AEP, Thornton JG, et al. (2020) Universal third-trimester ultrasonic screening using fetal macrosomia in the prediction of adverse perinatal outcome: A systematic review and meta-analysis of diagnostic test accuracy. PLoS Med 17(10): e1003190