Performance of fetal ultrasound and magnetic resonance imaging in predicting birthweight according to the test-to-delivery interval: A cohort study

Association of magnetic resonance imaging-derived maternal and fetal parameters with shoulder dystocia: matched case-control study

OBJECTIVE

To assess the association of fetal body measurements and maternal pelvic measurements obtained using magnetic resonance imaging (MRI) with the incidence of shoulder dystocia.

METHODS

This was a retrospective, single-center, case-control study conducted between January 2015 and December 2022. Patients whose delivery was complicated by shoulder dystocia and who underwent fetal MRI in the third trimester were included in the case group. Patients without shoulder dystocia who were delivered normally and who also underwent fetal MRI in the third trimester were included in the control group. Cases of multiple pregnancy, planned or emergency Cesarean delivery, fetal malformation or those with incomplete MRI examination were excluded. The case group was matched with the control group in a 1:2 ratio according to maternal age, maternal body mass index, gestational diabetes mellitus, diabetes mellitus Type 1 or 2, gestational age at MRI examination, gestational age at birth and birth weight. Shoulder dystocia was defined as per the Royal College of Obstetricians and Gynecologists and significant shoulder dystocia was defined as shoulder dystocia that was not resolved by the McRoberts' maneuver or suprapubic pressure. The following fetal and maternal measurements were quantified on MRI in both groups by two readers (one experienced and one inexperienced physician) who were blinded to the obstetric outcomes: fetal body volume (FBV), shoulder skin-to-skin distance, interhumeral distance, biparietal diameter (BPD), head circumference, obstetric conjugate (OC), sagittal outlet diameter (SOD), coccygeal pelvic outlet (CPO) and maximal transverse diameter (MTD). A stepwise backward logistic regression that included all measurements was performed. The inter-rater reliability of the measurements was estimated using interclass correlation coefficient (ICC). Statistical significance was set at P < 0.05.

RESULTS

Among the 1843 patients included in the study, there were 63 (3.4%) cases of shoulder dystocia. After matching, the case group comprised 36 patients and the control group comprised 72 patients. Patients who had shoulder dystocia, compared to those without, had higher FBV (P = 0.023), higher shoulder skin-to-skin distance (P = 0.003), lower OC (P = 0.021), lower SOD (P = 0.004), lower CPO (P = 0.045) and lower MTD (P = 0.001) in comparison with those without. The logistic regression model showed that FBV, shoulder skin-to-skin distance, BPD, SOD and MTD were independent predictors of shoulder dystocia. The measurements of interest had moderate to excellent reliability when repeated by an inexperienced reader. In those who had non-significant shoulder dystocia, only shoulder skin-to-skin distance was significantly greater and OC was significantly lower in comparison with the control group, whereas in those who had significant shoulder dystocia, only SOD and MTD were significantly lower in comparison with the control group.

CONCLUSIONS

MRI-derived fetal size, fetal shoulder measurements and maternal pelvimetry are associated with shoulder dystocia. Future studies could incorporate these measurements into a reliable predictive model for shoulder dystocia. © 2025 International Society of Ultrasound in Obstetrics and Gynecology.

[Routine application of results of a randomised trial: performance of ultrasound in daily routine practice for screening fetal macrosomia]

OBJECTIVE

To evaluate the performance of ultrasound at 36 weeks of gestation for screening fetal macrosomia according to the criteria of the randomized "DAME" trial.

METHOD

Retrospective observational study over 2 years in a type 3 maternity hospital. The primary outcome was birth weight above the 97th percentile on Audipog birth weight curves. The sensitivity, specificity, positive and negative predictive value of ultrasound at 36 SA for predicting macrosomia were calculated. For each patient, the absolute value of the difference between actual and estimated weight [EPF difference=(Birth weight - Estimated weight)/Birth weight] was calculated. Among undetected macrosomic newborns (≥97th Audipog percentile), risk factors associated with misdiagnosis were investigated (false negatives versus true positives).

RESULTS

Ultrasound estimation of fetal weight at 36 SA to detect fetal macrosomia had a sensitivity of 43.7% and a specificity of 82.1%. With a positive predictive value for ultrasound of 36,9% and a negative predictive value of 85.8%. The mean absolute value of the difference between estimated and actual birth weight was 287g (95% CI [115-409]), with a mean relative difference of 6.8% (95% CI [3.0-11.3]). A relative difference of less than 10% between the estimated weight and the birth weight was found in 68.5% of the patients. Among macrosomic newborns (≥97th percentile), a third-trimester estimated fetal weight at or above the 90th percentile, along with a history of fetal macrosomia in a previous pregnancy, reduced the risk of false negatives in the screening ultrasound at 36 weeks of gestation.

CONCLUSION

Ultrasound performed at 36 weeks' gestation in a population targeted for suspected fetal macrosomia in routine practice has moderate sensitivity and positive predictive value. It would be timely for teams also applying this protocol to share their results, in order to verify whether or not our findings can be generalized to a larger scale.

A simulation study to assess the potential benefits of MRI-based fetal weight estimation as a second-line test for suspected macrosomia

OBJECTIVE

To simulate the outcomes of Boulvain's trial by using magnetic resonance imaging (MRI) for estimated fetal weight (EFW) as a second-line confirmatory imaging.

STUDY DESIGN

Data derived from the Boulvain's trial and the study PREMACRO (PREdict MACROsomia) were used to simulate a 1000-patient trial. Boulvain's trial compared induction of labor (IOL) to expectant management in suspected macrosomia, whereas PREMACRO study compared the performance of ultrasound-EFW (US-EFW) and MRI-EFW in the prediction of birthweight. The primary outcome was the incidence of significant shoulder dystocia (SD). Cesarean delivery (CD), hyperbilirubinemia (HB), and IOL at < 39 weeks of gestation (WG) were selected as secondary outcomes. A subgroup analysis of the Boulvain's trial was performed to estimate the incidence of the primary and secondary outcomes in the true positive and false positive groups for the two study arms. Sensitivity, specificity, positive and negative predictive values (PPV, NPV) for the prediction of macrosomia by MRI-EFW at 36 WG were calculated, and a decision tree was constructed for each outcome.

RESULTS

The PPV of US-EFW for the prediction of macrosomia in the PREMACRO trial was 56.3 %. MRI-EFW was superior to US-EFW as a predictive tool resulting in lower rates of induction for false-positive cases. Repeating Boulvain's trial using MRI-EFW as a second-line test would result in similar rates of SD (relative risk [RR]:0.36), CD (RR:0.84), and neonatal HB (RR:2.6), as in the original trial. Increasing the sensitivity and specificity of MRI-EFW resulted in a similar relative risk for SD as in Boulvain's trial, but with reduced rates of IOL < 39 WG, and improved the RR of CD in favor of IOL. We found an inverse relationship between IOL rate and incidence of SD for both US-EFW and MRI-EFW, although overall rates of IOL, CD, and neonatal HB would be lower with MRI-derived estimates of fetal weight.

CONCLUSION

The superior accuracy of MRI-EFW over US-EFW for the diagnosis of macrosomia could result in lower rates of IOL without compromising the relative advantages of the intervention but fails to demonstrate a significant benefit to justify a replication of the original trial using MRI-EFW as a second-line test.