156 reported as contractions per minute (221). Further technical details about feature extraction as well as pre-processing analysis can be found in Sammali et al . (2018) (224). Contraction Amplitude: Contraction amplitude features reflect the relative strength of uterine contraction. Amplitude of contraction was assessed by calculating the standard deviation of the strain signal in the longitudinal and radial directions from its frequency spectrum (Parseval’s theorem) (230). A higher value reflected stronger uterine contractions. Results are reported for contractions in the longitudinal direction, separately for the anterior and posterior uterine walls. Contraction Direction Uterine peristalsis is thought to propagate mainly in one of two directions: either fundus-to-cervix (F2C) or cervix-to-fundus (C2F). The contraction direction was estimated by analysis of the radial strain signal representation in the spatiotemporal frequency domain, where the spatial domain is intended along the longitudinal direction of the uterus(229). The ratio between the strain signal energy in the quadrants corresponding to the two propagation directions (C2F and F2C) provided a global measure of the dominant propagation direction in each wall (posterior and anterior), separately (229). Basically, a more positive value 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 which did not show a predominant direction, being either circular movement, or standing or opposing contractions. Contraction Velocity Velocity, being propagation speed of the peristaltic waves in a certain direction (C2F or F2C, in mm/sec), was calculated for movement in the anterior and posterior walls. This was again done by analysing the radial strain signal representation in the spatiotemporal frequency domain, where the spatial domain is intended along the longitudinal direction of the uterus (224). 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
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