Abstract： Objective To identify the morphological characteristics of pelvic diaphragm hiatus in pregnant women with stress urinary incontinence ( SUI) by transperineal three-dimensional (3-D) ultrasound.Methods From Oct.2008 to Mar.2009,145 pregnant women (third trimester group) at 37-41 weeks of gestation underwent transperineal 3-D ultrasound investigation at Department of Obstetrics and Gynecology,the Sixth People's Hospital,Shanghai Jiaotong University,including 38 pregnant women with stress urinary incontinence (SUI) and the other 107 non SUI pregnant women.In the mean time,50 normal nulliparous healthy women were chosen as control group.The morphological characteristics of pelvic diaphragm hiatus,the diameter of pelvic diaphragm hiatus,pubovisceral muscle thickness and genitohiatal and levator ani angle were measured at rest,on maximum Valsalva and maximum pelvic floor contraction by 3-D ultrasound,respectively.Results Loosen connective tissue and pubococcygeus avulsion were observed in some pregnant women at third trimester.The area of pelvic diaphragm hiatus were (15.2 ±1.9),(16.4 ± 2.0) and (13.6±1.9) cm2,pubovisceral muscle thickness were (0.72 ±0.11),(0.68 ±0.14) and(0.77 ±0.11) cm,levator ani angle were (60 ±8) °,(57±10) ° and (64 ± 14)° at rest,on maximum Valsalva and maximum pelvic floor contraction respectively.These parameters were significantly increased than those in control group[(11.2 ±2.6),(14.5 ±4.5) and (9.2 ±2.6) cm2; (0.66 ±0.10),(0.67 ± 0.14) and (0.71 ±0.14) cm; (50 ±4) °,(51 ±5) ° and (46 ±5)°]at three maneuvers,respectively ( P <0.05).And those parameters of the anteroposterior hiatal diameter,lateral hiatal diameter,perimeter of pelvic diaphragm hiatus and area of pelvic diaphragm hiatus in SUI pregnant women were increased than those in non SUI pregnant women at three maneuvers,respectively (P < 0.05 ).Pubovisceral muscle thickness in SUI pregnant women was significantly lower than that in non SUI pregnant women at maximum pelvic floor contraction (P < 0.05 ),but there were not significant difference between SUI and non SUI pregnant women at rest and on maximum Valsalva in pubovisceral muscle thickness and genitohiatal and levator ani angle (P >0.05).Conclusions Pelvic floor anatomic remodeling is identified in late pregnant women.When compared with non pregnant women,the loosen pelvic floor connective tissue and the bigger diameters of pelvic diaphragm are observed in late pregnant women.It is observed that the increased diameters of pelvic diaphragm and decreased thickness of pubovisceral muscle in later pregnant SUI women than those in non SUI pregnant women.