Objective To observe the morphological characteristics of pelvic diaphragm hiatus in postpartum women with different first delivery patterns by transperineal three-dimensional ultrasound. Methods Eighty-two postpartum women were selected, and transperineal three-dimensional ultrasound examinations were performed 6 to 12 weeks after delivery. Fifty-five cases underwent transvaginal delivery (transvaginal delivery group), and the other 27 Cesarean delivery (Cesarean delivery group). Cases in transvaginal delivery group were subdivided into spontaneous transvaginal delivery group (n=44) and forceps transvaginal delivery group (n=11). The morphological characteristics of pelvic diaphragm hiatus in each group were observed, and the parameters of pelvic diaphragm hiatus and maximum bladder neck mobility were measured at rest, with Valsalva maneuver and pelvic floor contraction, respectively. Results Transperineal three-dimensional ultrasound demonstrated that the structure of puborectal muscles on both sides of pelvic diaphragm hiatus was vague, and unilateral and bilateral puborectal muscle avulsion was observed in some cases. The structure inside pelvic diaphragm hiatus was vague, and bladder echo was observed in some cases. With pelvic floor contraction, the parameters of pelvic diaphragm hiatus in transvaginal delivery group were significantly larger than those in Cesarean delivery group (P<0.05). At rest and with Valsalva maneuver, there was no significant difference in the parameters of pelvic diaphragm hiatus between transvaginal delivery group and Cesarean delivery group (P>0.05). The maximum bladder neck mobility in transvaginal delivery group was significantly larger than that in Cesarean delivery group [（15.3±3.8）mm） vs （11.7±3.6）mm]（P<0.01）. There was no significant difference in the parameters of pelvic diaphragm hiatus between spontaneous transvaginal delivery group and forceps transvaginal delivery group （P>0.05）, while the maximum bladder neck mobility in forceps transvaginal delivery group was significantly larger than that in spontaneous transvaginal delivery group [（21.2±2.9）mm vs （16.1±3.0）mm]（P＜0.01）. Conclusion The diameters of pelvic diaphragm hiatus are similar in postpartum women with different delivery patterns at rest and with Valsalva maneuver. Transvaginal delivery may be associated with a significantly larger pelvic diaphragm hiatus with pelvic floor contraction, and forceps transvaginal delivery may lead to a greater degree of bladder neck mobility, which may be related to postpartum stress urinary incontinence.