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Fetal zone enlargement (FZE) on prenatal ultrasound indicates increased activity within the fetal adrenal gland. FZE predicts preterm birth, denoting a fetal component in the initiation of labor. We tested the hypothesis that FZE indicates the likelihood of successful induction of labor (IOL).
Prospective cohort study of singleton pregnancies scheduled for IOL for indications not associated with uterine activity or infection. Immediately prior to IOL, fetal adrenal gland dimensions were measured. Fetal zone ratio (FZR=adrenal gland fetal zone width /total adrenal gland width), transvaginal cervical length (CL) and Bishop’s score were recorded. IOL was managed at the discretion of the provider (vaginal misoprostol, intracervical foley balloon, intravenous oxytocin or combinations). Obstetric and neonatal data were collected. The primary outcome was route of delivery. FZR was compared in those with successful vaginal delivery versus failed induction (1st stage arrest >24 hours and >12 hours oxytocin after rupture of membranes, 2nd stage arrest after >2 hours of adequate maternal effort). The predictive value of FZR for successful IOL was analyzed using ROC curve.
Seventy-four prospective patients enrolled; 46 patients met strict image quality criteria. Median BMI was 35 (24-72) kg/m2. IOL was successful in 36 (78.2%). Of 10 (22%) delivered by cesarean section, failed IOL was diagnosed in 5. Neither mean Bishop’s score nor CL differed between groups(p>0.05). FZR was significantly higher (42±8%) in successful IOL versus failed IOL (29%±7,p=0.002). By ROC curve analysis, FZR >35% had sensitivity of 86% and specificity of 100% (AUC 0.922, p=0.07) in predicting successful IOL; PPV=100%, NPV=50%.
Enlargement of the fetal zone of the fetal adrenal gland indicates fetal readiness for labor and predicts the success of IOL. With further refinement, this may provide a reliable tool for selecting IOL candidates.