To analyse the effects of the mode of delivery on the pelvic floor of the area of levator ani muscle hiatus in women after a second delivery.

A total of 1606 parturients who had a second delivery at Hangzhou Women's Hospital from March 2018 to February 2019 were subjected to postpartum translabial pelvic floor three-dimensional ultrasound examination from 42-60 days post-delivery to measure the levator ani muscle hiatus area at resting time and after maximum Valsalva maneuver. The patients were divided into four groups according to the mode of first and second delivery. Group 1 (1006 cases) had two vaginal deliveries, group 2 (485 cases) underwent two cesarean sections, group 3 (19 cases) had the first cesarean section and the second vaginal delivery, and group 4 (96 cases) had the first child delivered by vagina and second by cesarean section. Kruskal-Wallis H test was used to compare the differences of levator ani hiatus area at resting state, that after Valsalva maneuver, and enlarged levator ani hiatus area among the four groups. Linear regression was used to adjust the interference of other factors, and the influence of delivery mode on the study outcome was analyzed.

Among the four groups, the area of the levator ani muscle hiatus of woman at rest was statistically different (H=92.32, P＜0.001), and it was the largest in group 3 (14.80 cm²). The levator ani hiatus areas after Valsalva maneuver in groups 1-4 were 18.32 (25.62, 21.70) cm², 15.10 (12.58, 17.99) cm², 21.79 (16.80, 21.33)cm², and16.76 (13.40, 20.39) cm², respectively; pairwise comparisons showed a statistically significant difference between group 1 and group 2 (Z=12.934, P＜0.001), group 1 and group 3 (Z=-2.030, P=0.042), group 1 and group 4 (Z=-2.958, P=0.003), group 2 and group 3 (Z=-4.654, P＜0.001), group 2 and group 4 (Z=-3.128, P=0.002), and group 3 and group 4 (Z=-2.802, P=0.005). The increase in the area of levator ani muscle hiatus after Valsalva maneuver was statistically different among the four groups (H=121.15, P＜0.001). The increased area of the levator ani muscle hiatus was the largest (5.76 cm²) in group 3, followed by group 1 (5.44 cm²), and the smallest in group 2 (3.32 cm²). Compared with normal delivery, the area of the levator ani hiatus was significantly reduced by 1.22 cm² at the resting state (t=-4.681, P＜0.001) and by 2.10 cm² after Valsalva operation (t=-4.432, P＜0.001); the increase in the area of the levator ani muscle hiatus decreased by 0.86 cm², and the difference was statistically significant (t=-2.446, P=0.015).

The area of the levator ani hiatus is the largest in women with the first cesarean section and the second vaginal delivery, followed by women who have two vaginal deliveries. The area of the levator ani hiatus after Valsalva maneuver is the best indicator for evaluating levator ani hiatus.