During pregnancy, a woman experiences physical changes due to hormonal fluctuations, physiological variations, and anatomical modifications. The softening and increased compressibility of the tissues in the pelvic area may result in reduced stiffness of the supporting structures and increased vaginal distensibility. Changes can also occur in the pelvic floor geometry, with gradual enlargement of the levator hiatus occurring over pregnancy. 

The aims of this study are to measure levator ani muscle stiffness and force at three stages of the childbearing year, and to explore possible contributors to these changes. Preliminary results are reported.

This was an observational prospective study. Nulliparous women aged between 18 to 45 years of age with a singleton low risk pregnancy were invited to participate via advertisements. Exclusion criteria included: non-conversational level of English; more than two previous miscarriages after 16 weeks’ gestation; any risk factors developed during pregnancy; or planned elective caesarean. Local ethics committee approval was obtained.

Data were collected at three time points during the childbearing year: 18 to 24 weeks; 35 to 38 weeks; and 12 to 28 weeks post-partum. Demographic data included age, Body Mass Index (BMI), ethnicity, Beighton hypermobility score (1). Levator ani muscle stiffness and force data were measured using an elastometer, and transperineal 3/4D ultrasound captured levator ani muscle geometry post-partum. 

The elastometer is a hand-held computer controlled instrumented speculum that measures force at varying lengths of displacement (30 mm to 50 mm aperture, in a transverse direction). Measurement protocols developed previously (2) were modified to enable both stiffness and active force to be calculated. Measurements occurred at a sampling rate of 100 Hz, in ten, ten-second step-phase cycles. Within each cycle were relaxation and maximum voluntary contraction periods. Two sessions of measurement data were collected over a total of 5 minutes, where session one is considered a pre-conditioning and familiarisation cycle for the participant and not used in data analysis.

Muscle stiffness was calculated from the slope of the force-displacement curve measured from the most linear portion of the curve, between apertures 40 mm to 50 mm. Averaged force and displacement measures were used from the final one second of relaxation time. Three different force values were calculated at each aperture: total force, passive force, and active force. Active force was the difference between the peak force and the passive force. 

All participants were instructed on how to relax their pelvic floor muscles and how to perform an effective pelvic floor muscle contraction. Confirmation of participants’ ability to achieve this was via vaginal examination prior to the elastometry measurements.

Repeated measures ANOVA and pairwise comparisons, compared elastometry measurements at the three time periods. Age, BMI, ethnicity and Beighton scores were explored as possible explanatory variables for the elastometry measures as they were measured at each time point. Multivariant logistic regressions were used to explore the relationships between these variables.

A total of 52 women aged 18 to 43 years of age were accepted into the study, with post-partum measures completed on 45 participants. See Table One for demographics and clinical characteristics. Preliminary analysis of the results for this abstract focus on the elastometry measurements.

Average muscle stiffness was measured as 690 N/m ± 180 N/m at 18 to 24 weeks, 638 N/m ± 169 N/m at 35 to 38 weeks, and 668 N/m ± 232 N/m post-partum. However, these differences were not statistically significantly different, F(1.51, 40.74) = 0.93, p = 0.377.

Active force at 50 mm aperture decreased progressively from the initial assessment in the second trimester to the post-partum assessment. Initial measures of active force were 8.2 N ± 4.0 N at 18 to 24 weeks, reducing to 7.3 N ± 3.5 N at 35 to 38 weeks, to final post-partum measurements of 6.4 N ± 3.1 N. The differences were statistically significant, F(1.75, 41.91) = 7.69, p = 0.002. In pairwise comparisons, the difference was significant between 18 to 24 weeks and post-partum, a decrease of 1.8 N (95% CI, 0.46 to 3.1), p = 0.006. This represents a 22 % reduction in active force over the childbearing year (Figure One).

Multivariate regression modelling was completed for each time point to investigate the effect of potential explanatory variables on the response variables (stiffness and active force). At 18 to 24 weeks, an increase in passive force of 1 N predicts a corresponding increase in muscle stiffness of 34.0 N/m, p < 0.001, while an increase in active force of 1 N is predictive of a decrease in muscle stiffness of 7.5 N/m, p = 0.045. At 35 to 38 weeks, an increase in passive force of 1 N predicts a corresponding increase in muscle stiffness of 47.5 N/m, p < 0.001. Post-partum, an increase of the following variables were predictive of an increase in muscle stiffness: passive force of 1 N (increase of 60.7 N/m, p < 0.001); increase of 1 year of age (increase of 10.8 N/m, p = 0.004); and levator hiatal area increase of 1 cm2 (increase of 20.1 N/m, p = 0.013). The following variables were predictive of a decrease in muscle stiffness: BMI increase by 1 kg/m2 (decrease of 10.3 N/m, p = 0.026); and levator hiatal area increased by 1 cm2 on maximum pelvic floor muscle contraction (decrease of 29.3 N/m, p = 0.002). 

None of the chosen explanatory variables demonstrated any effect on active force at any of the time points.

Pelvic floor muscle property changes occur during pregnancy, as well as post-partum. There is a significant reduction in active force during pregnancy and post-partum. However preliminary analysis using age, BMI, Beighton hypermobility score, passive force, and stiffness did not demonstrate these variables as predictors for active force. Passive force influence on muscle stiffness almost doubled from early pregnancy to post-partum. Active force appears to have a negative effect on stiffness only in the 18 to 24 week time period. However, 3D ultrasound measures of levator hiatal area during contraction indicate a continued negative association of active force with stiffness post-partum.

Measurable changes occur to the pelvic floor muscles throughout the childbearing year, with active force demonstrating long term effects postpartum. It is unknown at this stage if pelvic floor muscle training during pregnancy would improve the retention of contractile force both during the pregnancy, and post-partum. Further analysis is planned with delivery outcomes and subjective data collected during the study.

Beighton, P. H., Solomon, L., & Soskolne, C. L. (1973). Articular mobility in an African population. Annals of the Rheumatic Diseases, 32(5), 413.
Kruger, J. A., Nielsen, P. M. F., Budgett, S. C., & Taberner, A. J. (2015). An automated hand-held elastometer for quantifying the passive stiffness of the levator ani muscle in women. Neurourology and Urodynamics, 34(2), 133-138. 10.1002/nau.22537