Abstract: Objective：To observe the variation and clinical significance of the angle of progression（AOP） and the midline angle（MLA） in stage of labor. Methods：Transperineal ultrasound were performed in 97 single cephalic term pregnancies. AOP and MLA were performed when the cervix enlarged at 2-3 cm，4-5 cm，7-8 cm and 9-10 cm. We analyzed the relationship between AOP and MLA. Results：Along with the birth process, the AOP is gradually increasing while the MLA is reduced gradually. There was a negative correlation between AOP and MLA. Conclusions：The transperineal ultrasound can be used in monitoring of the stages of labor，and exploite the advantages of noninvasiveness，convenience，reduce infection and repeatability.

Key words: Perineum, Ultrasonography, prenatal, Labor, obstetric, Angle of progression