## 2 Materials and Methods
### Placement Procedure  
The patients were placed in the lithotomy position, where they remained for the whole 10 min of the procedure, and they were advised not to move. A vaginal examination was performed using an open-sided Pederson speculum. The cervical os was then carefully cleaned with a sterile cotton swab and immersed in antiseptic solution. A disposable stiff outer-sheath Wallace catheter (Wallace(r) Classic) with a hard-guided curved probe and a rounded bulbed tip was used to facilitate insertion into the cervix and progress towards the uterine fundus (CooperSurgical Inc., Trumbull, CT, USA), as is performed in embryo transfer, after in vitro fertilization (IVF). The hard-guided probe was then removed, and the gas-sterilized EUG electrode array was inserted into the catheter until it reached the fundus. The sheath catheter was pulled back whilst maintaining the electrode array in place and in direct contact with the uterine walls. The physician verified that the tip of the electrode array was abutting the fundus by comparing the length of the electrode array inserted into the endometrial cavity with the length from the fundus to the cervix, derived from a previous TVUS examination, as well as from a 7 cm fixed marking labeled on the electrode. Furthermore, when the electrode was outside the endometrial cavity, the EMG  
\begin{table}
\begin{tabular}{l l} \hline \hline  & **Healthy Porous Women** & **(_n_ = 26)** \\ \hline Age & 28–48 \\ BMI & 17.3–32 \\ Number of children & 1–4 \\ Menstrual cycle duration (days) & 21–32 \\ Period duration (days) & 2–8 \\ History of Treated Past Pathologies & \\ Abortion, D\&C & 1 \\ Laparoscopic surgery for Endometriuma \& Endometriosis & 1 \\ Hystereoscopic Polypectomy & 1 \\ Myomectomy & 1 \\ Caesarean Section & 1 \\ \hline \hline \end{tabular} Number of participants is shown with the value of “_n_”. Values are displayed as minimum-maximum.  
\end{table}
Table 1: Demographic and gynecological/clinical characteristics of the healthy, porous participants.  
device monitor detected a high signal impedance (>50 k\(\Omega\)) and stopped recording from that channel, ensuring proper electrode placement. The Pederson speculum was then removed, and the electrode array terminal was connected to the recording device. The EUG recording was not started for three minutes to allow possible mechanically induced contractions to subside. The participants were questioned to verify that they did not feel any uterine contractions/cramps or any discomfort before initializing the EUG recording. No pain, discomfort, or bleeding were observed before starting the procedure. The electrode was compacted between the anterior and posterior uterine walls and therefore remained stable throughout the procedure. Artefacts were minimized by asking patients to remain immobile during the recordings. Movement artefacts were subsequently easily distinguished. After the end of the recording, the electrode array was removed from the uterine cavity. Figure 1 shows a TVUS recording after the electrode array placement inside the uterine cavity, with the mapping between the electrode array and its US image. No adverse events were identified or reported by the participants during or after the procedure.