Background: Preterm birth (PTB) remains a major cause of perinatal morbidity and mortality worldwide. Compared with term deliveries, early PTB (< 34 weeks' gestation) carries a 7-fold increased risk of neonatal death. Objective: To compare the Fetal Zone Depth (FZD) of fetal adrenal gland in term and preterm labor cases Patients and Methods: Thirty-three preterm pregnant women at 29-36 weeks of gestation with single pregnancy and clinical diagnosis of preterm labor and control group of 33 pregnant women at 37-40 weeks with term pregnancy were included in this study. FZD and Total Gland Depth (TGD) of fetal adrenal gland were ultrasonographically measured and FZD/TGD ratios were calculated and compared between the two groups. Results: No difference was found between the two groups in respect of age and number of pregnancies (P > 0.05). Yet, preterm birth history rupture of membrane, cervical dilatation and mode of delivery were higher in preterm group (P <0.05). Fetal adrenal gland FZD/TGD ratio was statistically significantly higher in preterm group compared to the term group (52% ± 5% vs. 29% ±9%; P <0.001). Conclusion: The growth in FZ as a fetal adaptation mechanism in increased fetal stress in preterm labor cases was at a significant level. Once supported by more comprehensive studies, we think that this result would be beneficial in the prediction of preterm labor in clinical practice.