## 4 Discussion
The VJZE-RMS, which correlates with the uterine contraction intensity, is grounded in the logical hypothesis that more synchronized depolarization in a larger volume of muscular tissue leads to higher voltage amplitudes. Statistically significant lower values for the LP phase of the menstrual cycle, in comparison to the EL and LL phases, align with findings in previous literature [8]. When directly comparing our results with those of [PERSON] et al., 2000, we noted that the VJZE-RMS, divided by a factor of ten, closely mirrors the Intrauterine Pressure (IUP) values, establishing an electrically derived IUP index. Similarly, the electrically derived uterine contractility index aligns with the uterine contractility measured with an IUP device and ultrasound (US) for three of the four menstrual cycle phases, without the need for constant multiplication or division. Figure 3 presents the comparison bar plots. For the nonpregnant uterus, only the JZE is responsible for the uterine contractions, while the two outer layers of the myometrium are responsible for the contractions during labor [4; 19; 20]. The contractile activity during menstruation, when the muscles are attempting to expel the endometrial tissue, resembles the behavior of the two outer layers during parturition [21]. Using external electrodes in a feasibility study for nonpregnant women, statistically significant differences were only found between menstruation and at least one of the LP, EL, and LL phases, but not among the non-menstruating phases [10]. Hence, it can be argued that external electrodes can only track the difference in contractility between the inner one-third of the myometrium and the outer two-thirds, which have significantly different characteristics. Regarding the contractility characteristics of the JZE, it is considered to have a finer and more wavelike contractility pattern, which is called peristalsis and more closely resembles the peristaltic activity of bowel movements [2]. The peristaltic activity of the uterine muscles during the menstrual cycle has also been correlated with the hormonal levels for nonpregnant women. After menstruation, the estrogen levels start to increase, showing a peak during days 8-12. The observed peak agrees with the peak of the uterine contractions, which, according to the literature, plays a significant role in the facilitation of sperm transport [2; 8; 22]. On the contrary, after ovulation, the estrogen levels show a downward trend and, at the same time, the progesterone levels show an upward trend, with a peak around seven days after ovulation. Furthermore, after ovulation and until the end of the menstrual cycle, the number of intrauterine contractions decreases compared to the periovulatory period, showing that progesterone inhibits the excessive peristalsis of the uterus, even though a second smaller estrogen peak is observed on around the seventh day after ovulation. Hence, it is obvious that progesterone dominates the peristaltic activity of the nonpregnant uterus during the luteal phase, creating a quiescent environment, which facilitates easier embryo implantation to the endometrium [2, 8, 22]. The EUG demonstrated the detection of changes in the peristaltic activity among the phases of the menstrual cycle that coincide with the hormonal changes of estrogen and progesterone. Furthermore, in a study measuring the electrical activity of extra-corporeal-perfused human uteri, the frequency of the rhythmic electrical activity was increased with the administration of a dose of 178-estradiol and was reduced with the administration of a high dose of progesterone [23]. These results further support our observations regarding the frequency changes in the JZE's electrical activity and provide an additional electrochemical validation of our results. When comparing the frequency- and amplitude-related features, the amplitude-related features are dependent on the impedance of the recording area.