The levator hiatus (LH) is an anatomical opening within the pelvic floor, formed by the levator ani muscles and traversed by the urethra, vagina, and rectum. The minimal levator hiatus (MLH), comprising the narrowest region of this opening, is bordered by the pubococcygeus and puboanalis muscles anteriorly, puborectalis laterally, and the levator plate posteriorly. It plays a key role in vaginal support and pelvic floor integrity. Three-dimensional endovaginal ultrasound (3D EVUS) has enabled detailed visualization and measurement of MLH anatomy. While some studies suggest an inverse relationship between MLH size and pelvic floor muscle (PFM) strength, others find no significant correlation. The inconsistency in findings is particularly pronounced in postpartum populations, where obstetric factors such as prolonged labor, perineal trauma, or instrumental delivery may impact pelvic floor function. Given the limited and conflicting evidence, this study focuses on primiparous women to investigate whether MLH size is associated with PFM strength, and whether this relationship is influenced by obstetric risk factors for levator ani injury.

This cross-sectional study included primiparous women stratified by obstetric factors into high-risk and low-risk groups. The high-risk group was defined as having any of the following risk factors: prolonged second-stage labor (>160 minutes), operative vaginal delivery, fetal head circumference >35.5 cm or perineal laceration ≥ second degree. At 6-12 weeks postpartum, MLH size (cm2) was measured using 3D Endovaginal Ultrasound (EVUS), while PFM strength (mm Hg) was assessed using the Peritron™ perineometer during maximal voluntary contraction. These measurements were made separately in low-risk and high-risk groups. Analysis was performed with Python version 3.11.8. Means were compared using student t-test. Pearson correlation analysis was performed to assess the relationship between increasing MLH size and decreasing PFM strength. This relationship was compared between risk groups.

159 postpartum primiparous women were recruited, 81 in the high-risk and 78 in the low-risk group. The mean PFM strength in the high-risk group was 21.28 mm Hg (SD = 12.75), and in the low-risk group was 21.50 mm Hg (SD = 12.18). The difference in PFM strength between the two groups was not statistically significant (t = -0.11, p = 0.913). Mean MLH size did not differ significantly between the high-risk (11.19 ± 1.95 cm²) and low-risk (11.58 ± 2.02 cm²) groups (t = -1.24, p = 0.218). There were no significant differences between the two groups across age, BMI, and race. No significant correlation was observed between MLH size and PFM strength in either the low-risk (r = -0.048, p = 0.681) or high-risk group (r = -0.002, p = 0.988). Overall, we found that increasing MLH size did not significantly correlate with decreasing PFM strength in low vs high-risk obstetric groups.

Our findings demonstrate that PFM strength and MLH size were not significantly impacted by obstetric risk factors, nor were they correlated in the early postpartum period. These findings suggest that obstetric history alone may not reliably predict early pelvic floor dysfunction. The absence of early associations underscores the value of ongoing longitudinal research to determine whether these relationships evolve over time and to better understand the progression of pelvic floor recovery and dysfunction beyond the immediate postpartum phase.