117 INTRODUCTION: Worldwide, approximately one in six couples experiences infertility during their reproductive ages (from 20 to 44 years old) [1]. Most women with (sub)fertility problems seek clinical support from assisted reproductive technologies, such as in vitro fertilization (IVF). In the last decade, the number of IVF cycles performed every year has increased by over 20% [2], [3]. However, the success rate of IVF treatment remains below 30%, with only a 4% increase [2], [3]. An IVF cycle consists of the preparation of the patient with exogenous hormones, after which the developed oocytes (eggs) are retrieved from the ovarian follicles and fertilized in vitro. The resulting embryos are then transferred back into the uterine cavity. Subsequent successful implantation of the embryo in the uterine wall leads to pregnancy. Several studies indicate that dysfunction of uterine contractility is one of the possible reasons hampering successful embryo implantation [4]–[7]. The uterine body consists of three parts: an outer serosal layer, an inner lining called endometrium, and an intermediate muscle layer called the myometrium (see Figure 5.1). Uterine contraction of a nonpregnant uterus, which refers to the shortening of the uterine muscle, was first mentioned in 1937 by Dickinson based on bimanual palpation [8]. Uterine contraction is reported to mostly occur around the endometrium, acting as a wave propagating alongside the endometrium. The resulting rhythmic uterine deformation (motion) is also known as uterine peristalsis (UP) [7]. Due to the influence of hormone levels, the UP patterns change in terms of direction, frequency, velocity, and amplitude during different phases of the menstrual cycle. In particular, during the luteal phase, when opposing contraction waves are often generated to keep the embryos inside the endometrial cavity and facilitate their implantation. Women suffering from infertility problems are also likely to suffer from uterine disorders, such as endometriosis and adenomyosis, or endocrine disbalances, which can affect UP and hamper embryo implantation. Therefore, a reliable assessment of the uterine activity outside pregnancy can be expected to provide valuable insight into the influence of UP on IVF failure.
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