203 represented movement predominantly in the F2C direction, whereas a more negative value represented movement predominantly in the C2F direction. A value around zero represented movement without a predominant direction, that being opposing contractions. Contraction Velocity: Velocity, which is propagation speed of the peristaltic waves in a certain direction (C2F or F2C, in mm/sec), was calculated. This was again done by analysing the radial strain signal in the spatiotemporal frequency domain, along the longitudinal direction of the uterus (229). The analysis was performed over a window of 20 seconds sliding over the full recording time. Subsequently, the median velocities (MV) in C2F and F2C directions were calculated by averaging velocities over time in the corresponding directions. A high value reflected increased velocity in the reported direction. Contraction Coordination: We also assessed the coordination of uterine contractions. In order to quantify this, we assessed whether the anterior and posterior wall of the uterus were moving synchronously or asynchronously to each other. This resulted in a feature defining the uterine contraction coordination depending on the similarity in the dominant contraction direction between the anterior and posterior walls, denoted as: mean square error (MSE). 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 (UC) features: - 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 (MSE)
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