© TÜBİTAK.Background/aim: This study aimed to evaluate the relationship between second-and third-trimester clavicle lengths and birth weight and shoulder dystocia. Materials and methods: This prospective observational study included 181 patients who presented to the Private Etlik Lokman Hekim Hospital for routine pregnancy visits between March 2019 and March 2020. In addition to routine pregnancy examinations, the patients also underwent ultrasonography twice at weeks 20–23 and 33–36 to determine the length of the fetal clavicle. The patients were evaluated for shoulder dystocia in the second stage of labor. The birth weight of the neonates was recorded. The primary objective of this study was to establish the relationship between third-trimester clavicle length and shoulder dystocia. Results: Fetal clavicle length increases in the second trimester with the advancing gestational week but does not significantly change in the third trimester. We did not observe any significant difference for second trimester clavicle length between the type of delivery, birth weight, or shoulder dystocia. Moreover, we did not observe any significant difference for third trimester clavicle length between types of delivery. However, we found a significant relationship between third trimester clavicle length and birth weight and shoulder dystocia. The median third-trimester clavicle length was 39.5 mm (range: 30.7–43.9) in neonates who did not develop shoulder dystocia and 42.5 mm (range: 41.4–43.1) in the 3 neonates who developed shoulder dystocia. The third-trimester clavicle length cut-off for shoulder dystocia was calculated as 41.35 mm (sensitivity: 100.00%, specificity: 83.82%, accuracy: 84.5%). The third-trimester clavicle length cut-off for macrosomia (defined as birth weight of ≥4100 g) was 40.75 mm (sensitivity: 87.50%, specificity: 77.56%, accuracy: 78.05%). Conclusion: Third-trimester fetal clavicle length, an important component of biacromial diameter, as measured by ultrasonography, is a practical and significant method for predicting macrosomia and shoulder dystocia.