Introduction and Hypothesis <p>Stress urinary incontinence (SUI) may be related to dysfunction of the core stabilizing system. We hypothesized that women with more severe SUI would demonstrate reduced abdominal-muscle thickness and decreased lumbopelvic control. This study investigated differences in abdominal-muscle thickness and lumbopelvic control among women with different SUI severities compared with continent women.</p> Methods <p>This cross-sectional study included 54 women: 21 women with mild SUI, 12 women with moderate SUI, and 21 healthy controls. Abdominal-muscle thickness was measured using ultrasonography. Lumbopelvic control was assessed using one- and double-leg-lowering tests. One-way analysis of variance and post hoc tests were used for data analysis.</p> Results <p>Women with both mild and moderate SUI demonstrated significantly reduced thickness of the transversus abdominis (<i>p</i> &lt; 0.01), internal oblique (<i>p</i> &lt; 0.01), and external oblique muscles (<i>p</i> &lt; 0.01). The SUI groups also exhibited lower angles in the one-leg-lowering test (control = 67.04 ± 20.78°; mild SUI = 46.25 ± 31.43°; moderate SUI = 34.31 ± 32.10°; <i>p</i> &lt; 0.01) and the double-leg-lowering test (control = 45.19 ± 24.56°; mild SUI = 28.96 ± 23.90°; moderate SUI = 24.38 ± 26.17°; <i>p</i> = 0.04).</p> Conclusions <p>Women with SUI demonstrated significantly reduced abdominal-muscle thickness and lower performance on lumbopelvic control tests than continent women, with more pronounced deficits in moderate SUI. These findings highlight alterations in the core stabilizing system associated with SUI severity. Comprehensive rehabilitation approaches addressing both pelvic-floor and core-muscle function may be beneficial.</p>