Objective Observation of the pelvic floor structure in the third trimester of pregnancy by three dimensional ultrasound, and to explore the influence of pregnancy on the anatomical structure of the pelvic floor. Methods Observed the Pelvic floor structure by the transperineal two dimensional and three dimensional Sonographic examination ，which in normal group (106 cases) and in abnormal group(20 cases)of pelvic floor function. During the Valsalva movement,measure the area of levator hiatus (cm2) via dynamic three-dimensional,and to observe the morphology of the anal sphincter by Ultrasonic tomography. Results The area of levator hiatus in the Pelvic floor dysfunction group were significantly higher than that in the normal group (P < 0.05). Conclustion We consider that the area of levator hiatus can better reflect the changes in the structure of women''s pelvic floor during pregnancy,and be useful to evaluate the injury of the levator hiatus.