192 method used to assess uterine peristalsis. This, however, does make the results of the various studies difficult to compare. As seen in Tables 7.3 and 7.4, four techniques of visualising uterine peristalsis have been used, each with its own limitations. Few can assess all potential parameters and results are often based on subjective assessment of obtained images. To ensure comparability amongst studies, it would be better to eradicate this subjectivity by automating the analysis, as proposed by Sammali et al. (2019) (229,260). Studies were also difficult to compare due to heterogeneity in study designs, populations and intervention. The moment of measurement in the menstrual cycle seemed to be of major influence on the observed contraction features, however not all studies reported this (253). Additionally, the sample size of most of the included studies was small. In three studies, participants were divided over the menstrual phases and uterine contractions were only assessed in that particular phase (245,250,252). As the result of this, only a few participants were assessed per menstrual phase. One might argue whether this is enough to draw conclusions. Furthermore, some included studies are relatively dated. This results from a lack of recent literature on the influence of uterine abnormalities, specifically congenital uterine anomalies, on uterine peristalsis. Besides, reported differences in uterine contractions in endometriosis patients might be influenced by an unreported presence of adenomyosis. Since the sonographic diagnosis of adenomyosis can be difficult, an undiagnosed presence in this endometriosis group could have biased the outcome. Additionally, an article on the influence of endometriosis, presented as endometrial cysts, was included. It could be questioned if this has a comparable effect on uterine peristalsis as in (deep) endometriosis or adenomyosis. Finally, the included studies use different definitions of uterine peristalsis. Nonpropagating contractions were included in the definition of uterine peristalsis in some, and defined as dysperistalsis, but excluded or unmentioned in other studies. This calls for a standard definition for uterine peristalsis and its various features.
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