Changes in Human Pelvic Floor Muscle Geometry After Pregnancy and Parturition

Objectives: Maternal pelvic tissues undergo remodeling in pregnancy and are often injured by the mechanically demanding task of delivering a fetus vaginally. While the effects of pregnancy and childbirth on the pelvic floor muscles (PFMs) have been observed in cross-sectional and animal studies, longitudinal human data encompassing pregnancy and parturition are sparse. We used longitudinal human magnetic resonance imaging (MRI) acquired prospectively to quantify the impact of the first pregnancy and mode of delivery on PFM geometry.

Methods: In this prospective longitudinal study, 135 nulliparous cis-women planning pregnancy between 2008 and 2013 underwent standardized evaluations, including high-resolution T2-weighted MRI, at the first visit (Vnp). Participants who became pregnant and gave birth underwent the same evaluations at 6-months (Vpp) postpartum. PFMs were segmented manually, smoothed, and quantified by measuring surface-to-surface distances (STSDs) between aligned Vnp and Vpp geometries. Regional variation was visualized with color maps that highlighted PFM regions with large STSDs. The percentage of STSDs for each individual considered small ( < 1mm) and large (>10mm) were also calculated. Independent t-tests determined if PFM alterations from Vnp to Vpp while independent t-tests and chi-square tests determined if demographics differed between participants who had a Cesarean section (CS) versus a spontaneous vaginal delivery (SVD).

Results: Demographics (e.g., BMI, gestational age at delivery) did not differ between CS and SVD groups. All individuals included thus far in this ongoing analysis exhibited changes from Vnp to Vpp. In our preliminary analysis, we found that STSDs significantly differed between SVD (n=9) and CS (n=5) groups (p < 0.001, Fig.1A). STSDs varied regionally, with the majority exhibiting large STSDs regions that encompassed portions of the pubovisceralis and iliococcygeus muscles (n=9, 6 SVD and 3 CS). The minimum STSD for each individual was < 0.001mm, the median between 1.35mm-2.66mm, and the maximum between 8.31mm-18.95mm. While the median SVD STSD was smaller, this group had the largest STSDs. Though not statistically significant, the SVD group contained the individuals with the largest percentage of small and large STSDs (Fig.1B).

Conclusions: This study revealed that pregnancy and parturition have a heterogenous impact on PFM structure, with mode of delivery contributing to differences observed between pre-pregnancy and postpartum PFM geometries. Though the median STSDs were relatively small, a wide range in values was observed with regions of greatest differences varying in location. While this analysis is limited by its sample size, it is strengthened by the inclusion of prospective longitudinal data. Ongoing efforts are focused on incorporating the entire data set, quantifying observed region-specific changes, and correlating results with all relevant demographic variables.

Figure 1: (A) Violin plots displaying all STSDs for each group. (B) Boxplots displaying the percentage of STSDs that were considered small ( < 1mm, bottom) and large (>10mm, top) for each individual.