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.