Imaging studies characterizing levator ani muscle (LAM) injury after childbirth are limited. There is a strong positive correlation between vaginal prolapse stage and the minimal levator hiatus (MLH) measurement that can be seen on 3D Endovaginal Ultrasound (EVUS).  Also, obstetric levator avulsion is associated with larger levator hiatus and wider genital hiatus after delivery based on the use of 3D transperineal US. EVUS studies of morphologic differences between high and low risk obstetric populations are lacking.

This was a single-site prospective cohort study of patients undergoing their first vaginal delivery at a university hospital. Subjects were categorized as high risk if they had a ≥ 2nd degree perineal laceration, a prolonged second stage of labor >160 min, instrumented delivery, or fetal head circumference >35.5cm. Women were classified as low risk if they had an intact perineum and no previously defined risk factors for LAM injury at vaginal birth. Relevant obstetric information was obtained by chart abstraction. At the 6-week postpartum visit, a 3D EVUS was obtained. Axial images were measured for total MLH area (cm2), length (mm), and width (mm). Patients also completed PFIQ-7 and PFDI-20 questionnaires at 12 weeks postpartum.

There was no difference in the MLH measurements between the high and low risk groups (p=0.16). When evaluating the correlation between MLH measurements of area, width and length with pelvic floor dysfunction (PFIQ-7 and PFDI-20 total scores) stratified by high and low risk, these correlations were not significant (all p-values >0.05). The correlations between MLH area and total PFIQ and PFDI score in the high-risk group were 0.08 and -0.18, respectively. In the low-risk group, the correlations between MLH area and PFIQ and PFDI scores were 0.026 and 0.083, respectively. When adjusting for confounding factors (age, BMI, smoking status, fetal weight, and time since delivery), there was no significant difference in the US measurements when comparing high and low-risk obstetric groups (p-values >0.5).

In the postpartum period, there is no significant association between pelvic floor morphology (specifically MLH) and pelvic floor dysfunction symptoms when stratified by low versus high-risk obstetric groups. MLH was not a predictor of pelvic floor dysfunction in women with high risk obstetric factors in the postpartum period.