The FL/AC ratio for prediction of shoulder dystocia in women with gestational diabetes

Purpose: To determine if sonographic variables, including fetal femur length to abdominal circumference (FL/AC) ratio, are associated with shoulder dystocia in women with gestational diabetes. Methods: This was a retrospective cohort study of women with gestational diabetes who delivered singleton infants at Parkland Hospital from 1997 to 2015. Diagnosis and treatment of gestational diabetes were uniform including sonography at 32–36 weeks. Biometric calculations were evaluated for correlation with shoulder dystocia. Results: During the study period, 6952 women with gestational diabetes underwent a sonogram at a mean gestation of 34.8 ± 1.8 weeks. Of 4183 vaginal deliveries, 66 experienced shoulder dystocia (16/1000). The FL/AC was associated with shoulder dystocia (p < 0.001) with an AUC of 0.70 (95% CI: 0.64–0.77). This was similar to age-adjusted AC and head circumference to AC ratio (HC/AC) (both with an AUC of 0.72). All other measurements, including estimated fetal weight, were inferior. When examining the 257 women with multiple sonograms after 32 weeks’ gestation, FL/AC was stable with advancing gestational age (p = 0.54) whereas age-adjusted AC and HC/AC were not (p < 0.001). Conclusions: The FL/AC is associated with shoulder dystocia in women with gestational diabetes. Additionally, it is a simple ratio that is independent of the reference used and remains stable, unlike age-adjusted AC and HC/AC ratio.