## 1 Introduction  
The junctional zone endometrium (JZE) is the layer between the endometrium and myometrium. It is described as a compaction of smooth muscle fibers surrounded by little extracellular matrix. The JZE is better visualized and evaluated using T2-weighted Magnetic Resonance Imaging (MRI) and 3D Transvaginal Ultrasound Scanning (TVUS) [1]. Current scientific data provide evidence that the JZE is an important part of the nongravid uterus anatomy, structure, and functionality. During the late proliferative and luteal phases, the myometrium is in relaxation and the JZE contracts regularly [1,2]. The JZEthickness and contractility alterations have been associated with myometrial and sub-endometrial pathologies like submucosal fibroids and adenomyosis [3]. Additionally, when the JZE exceeds 12 mm in thickness, this is considered to be a bad prognostic factor for the implantation potential [2]. The JZE's contractility frequency is adjusted by the ovarian hormones' cyclic activity and is also altered by pathologies adjacent to it, such as fibroids and polyps. Hence, the appearance, thickness, and contractility of the JZE can be used as markers for the diagnosis and prognosis of normal and abnormal uterine functioning, the implantation potential, early pregnancy loss, and probably for the early detection of endometrial cancer [1; 2; 3].  
Currently, the uterine JZE contractility has only been monitored within the research context via TVUS and MRI. MRI evaluation of the JZE allows for better imaging acquisition and has a higher diagnostic accuracy with lower interobserver variation than TVUS. However, both techniques are complex and require costly machinery and highly experienced operators, whilst ultrasound lacks reproducibility [1; 2; 3]. Hence, a simple, reliable, low-cost tool for the objective and accurate assessment of uterine contractility is missing [4].  
As in every muscle, the contraction of uterine muscles is generated due to the propagation of electrical biosignals, which potentially play a significant role with regards to fertility and various benign and malignant conditions [5]. Currently, there is no standard method for directly recording electrical biosignals from inside the endometrium without affecting the measurand. In an in vivo attempt using intrauterine electrodes, the administration of sedatives was necessary, especially to nulliparous women, to allow them to withstand the potential pain and discomfort of the procedure [6]. Furthermore, due to the invasiveness of the procedure, there is an increased probability of secondary contractility effects due to the introduced mechanical pressure that is applied to the uterus from the inserted electrodes, affecting the normal uterine peristaltic activity and leading to measurement artefacts. Alternative methods that involve placing sensors on the abdomen have shown promising results for pregnant uteri [7]. However, when using the same technique for nonpregnant uteri, even though fine peristaltic movements with contractility variations across the menstrual cycle have been observed, no significant differences outside menstruation have been noted [8; 9; 10].  
Recently, we developed the ElectroUteroGraph (EUG), a methodology with technical requirements for the direct recording of electrical biosignals from the endometrial cavities of nonpregnant women using a low-profile intrauterine electrode array [11]. The procedure mimics the procedure of embryo transfer, whereby an outer-sheath catheter with a stiff guide wire is used for the placement of the electrode array. Additionally, the electrode placement and the measurement technique are simple, standardized, and pain-free. In this article, we focus on our clinical findings measuring the JZE's electrical activity across the menstrual cycle for parous nonpregnant women using the EUG device.