157 reported direction. Results were reported separately for the anterior and posterior uterine walls. Contraction Coordination: In addition to the features described above, we also aimed to assess the coordination of uterine contraction. This is the first attempt at a quantifiable measurement of coordination of uterine movement up to now. In order to quantify this, we assessed whether the anterior and posterior wall of the uterus were moving synchronously or asynchronously. This was accomplished by estimating the time evolution of the estimated propagation direction over the anterior and posterior wall using a running window of 20 seconds. The two resulting evolutions were then compared using a similarity measure. This resulted in a feature defining the uterine contraction coordination depending on the adopted similarity measure: mean square error (MSE). Two additional coordination features (Hausdorff distance metric and cross correlation) were also investigated, and are shown in the appendix A. Again, full details on the technical background of these units has been published elsewhere (229). A higher value reflected decreased contraction coordination. Study outcomes: The primary outcomes investigated were the following uterine contraction features, compared between the four menstrual phases. - Frequency, in contractions/minute - Amplitude (unitless) - Direction, (unitless, whereby >0.0 globally represents F2C movement, and <0.0 represents C2F movement) - Median Velocity (mm/sec) - Coordination, in Mean squared error Statistical methods: Statistical analysis was performed using IBM SPSS statistics version 27. The Shapiro-Wilk test was first employed to test the normality of the distributions. Comparison of the outcome measures (frequency, amplitude, direction, coordination and velocity) between the various phases was done using the Kruskal-Wallis test if abnormally distributed, and a one-way ANOVA if normally distributed (with Bonferroni correction). Statistical significance was defined as a p-value < 0.05. This study is reported according to STROBE guidelines (189)
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