Objective and comprehensive characterization of uterine peristaltic activity throughout the menstrual cycle by means of intracavitary electrohysterography, a cohort study.

Conceptualization, J. A.‐R., J. G.‐C.; Data curation, J.‐M. M.‐T., A. D.‐M., P. A.‐F., S. C.‐S., G. C.‐C., M. d.A.‐G.; Formal analysis, J.‐M. M.‐T., A. D.‐M.; Funding acquisition, J. A.‐R.; Investigation, P. A.‐F., S. C.‐S., G. C.‐C., M. d.A.‐G., J. L.‐A.; Methodology, J. A.‐R., J. G.‐C., P. A.‐F., S. C.‐S., G. C.‐C., M. d.A.‐G., J. L.‐A.; Project administration, J. A.‐R., P. A.‐F.; Resources, J. A.‐R., P. A.‐F., S. C.‐S., G. C.‐C.; Software, J.‐M. M.‐T., A. D.‐M.; Supervision, J. A.‐R., J. G.‐C.; Validation, J. A.‐R., P. A.‐F., S. C.‐S., G. C.‐C., M. d.A.‐G., J. L.‐A., J. G.‐C.; Visualization, J.‐M. M.‐T., A. D.‐M.; Writing—original draft, J.‐M. M.‐T., A. D.‐M.; Writing—review and editing, P. A.‐F., J. L.‐A., J. G.‐C.

The Institutional Ethics Review Committee approved the study on 29 March 2023 (Reference No. 2023‐108‐1). It was conducted in compliance with The International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH) Good Clinical Practice. 40

JMMT (DIN2021‐012073) and ADM (PTQ2024‐013671) report financial support from the Ministerio de Ciencia, Innovación y Universidades. JAR reports board membership and a licensed patent (EP4051114B1) with Sonda Devices S.L. JGC reports consulting and advisory services with Sonda Devices S.L. The funding sources had no role in the design, data collection, analysis, interpretation, or writing of the manuscript. All other authors declare no competing interests.

Forty healthy women with proven self‐fertility were included in the study, resulting in a total of 120 IC‐EHG records in the different phases of the cycle. After quality check, 95 IC‐EHG recordings from the uterine fundus and 90 from the lower segment of the uterus were deemed acceptable for inclusion in the study. A detailed summary of patient and record inclusion is shown in Figure  1 . Regarding patients' tolerability and safety of IC‐EHG, all participants were closely monitored throughout the study, and no adverse events or side effects were reported. No participants dropped out due to discomfort. The severity of pain experienced during insertion, recording, and removal of the probe was assessed using the visual analogue scale (VAS) in a subset of 30 records, with mean values of 1.62, 0.88, and 0.46, respectively. The gynecological and obstetric characteristics of the volunteers are summarized in Table  1 . The menstrual cycle day for each visit was calculated as the difference in days from the date of the last menstrual period to the date of the corresponding visit. Likewise, the LH+ day was obtained. Characteristics of included patients, expressed as mean ± standard deviation for normally distributed variables and median (interquartile range) for non‐normally distributed variables. Abbreviations: BMI, body mass index; ICO, internal cervical os. Figure  2 provides a representative example of IC‐EHG recordings from the fundus and lower segment of the uterus throughout the menstrual cycle (MF, EL, and LL phases). Peristalsis features derived from IC‐EHG are summarized in Figure  3 , revealing complex dynamics in uterine contractility, influenced by both the menstrual phase and regional myometrial differences. A comprehensive numerical description of these results is provided in Table  S1 . The detailed results of all comparisons between menstrual phases and uterine regions across all analyzed parameters, i.e. p ‐value, 95% confidence intervals of the mean differences and Cohen's d effect size, are provided in Tables  S2 and S3 , respectively. Example of IC‐EHG signals recorded during mid follicular (MF), early luteal (EL), and late luteal (LL) phases. Box‐and‐whisker plots representing uterine contraction (CT) and basal activity features extracted from IC‐EHG recordings across different phases of the menstrual cycle: Mid‐follicular (MF, red), early luteal (EL, green), and late luteal (LL, blue). Lighter‐colored boxes represent measurements from the uterine fundus (sample size: MF = 33, EL = 28, LL = 34), while darker‐colored boxes correspond to the lower segment (sample size: MF = 27, EL = 27, LL = 36) of the uterus. Statistically significant differences between phases are indicated by horizontal bars (Wilcoxon signed‐rank test, p ‐value <0.05), with gray bars denoting differences found in the uterine fundus and black bars representing those in the lower segment. Differences between uterine regions are highlighted with shaded areas. Uterine contraction frequency (Figure  3A ) was found highest during the EL phase (fundus: 3.91 IQR 0.45 CT/min, lower segment: 4.03 IQR 0.58 CT/min) and significantly lower during the MF phase (fundus: 3.28 IQR 1.08 CT/min, p  = 0.042; lower segment: 3.65 IQR 1.21 CT/min, p  = 0.024). Both uterine regions exhibited similar frequency patterns with no significant inter‐regional differences across any menstrual phase. Contraction amplitude (Figure  3B ) in the fundus showed a progressive decrease from MF (16.27 IQR 7.72 μV) to EL (14.06 IQR 6.57 μV, p  = 0.054), and from EL to LL (10.56 IQR 7.72 μV, p  = 0.014). In the lower uterine segment, amplitude was also lowest during LL (9.47 IQR 3.93 μV), significantly differing from both MF ( p  = 0.016) and EL ( p  = 0.018). Additionally, fundal contractions in the MF phase had significantly greater amplitude compared to the lower segment (12.38 IQR 6.44 μV, p  = 0.001). Basal amplitude (Figure  3C ) peaked during the EL phase (fundus: 5.97 IQR 2.15 μV; lower segment: 5.51 IQR 3.86 μV) and reached significantly lower minima in the LL phase for both regions (fundus: 4.55 IQR 3.07, p  = 0.028; lower segment: 3.66 IQR 1.58 μV, p  = 0.012). During MF, resting amplitude remained near maximal in the fundus, while in the lower segment it was significantly lower ( p  = 0.004). As shown in Figure  3D , the uterus remains in a contractile state for the longest percentage of time during the EL phase (fundus: 54.71 IQR 6.49%; lower segment: 56.86 IQR 10.40%), whereas it was shortest during the LL phase (fundus: 51.55 IQR 8.50%, p  = 0.007; lower segment: 48.91 IQR 7.83%, p  = 0.001). Notably, regional differences emerged, with the fundus exhibiting a significantly longer contraction time percentage than the lower segment during the MF ( p  = 0.003). Regarding the uterine contractility index (Figure  3E ), the highest values were observed in the fundus during the MF phase (531.37 IQR 314.49 μV·s/min), decreasing during EL (465.20 IQR 256.15 μV·s/min, p  = 0.068). It reached its minimum in LL (331.25 IQR 268.62 μV·s/min), with significant differences compared to MF ( p  = 0.001) and EL ( p  = 0.022). The lower segment showed values comparable to the fundus during the EL and LL phases but significantly lower during MF (307.49 IQR 263.01, p  = 0.003). Figure  3F illustrates that the highest levels of local coordination were observed in the EL phase for both uterine regions (fundus: 3.92 IQR 0.47; lower segment: 4.13 IQR 0.54). Significantly lower coordination values were obtained during the MF and LL phases. Across all phases, coordination in the fundus was significantly weaker than in the lower segment (MF p  < 0.001; EL p  = 0.006; LL p  = 0.002).

A deeper understanding of uterine electrophysiological and peristaltic characteristics across the menstrual cycle is crucial for advancing women's reproductive health. 2 , 3 , 13 , 27 Detailed analysis of uterine contractility and its inherent regulatory mechanisms can contribute to understanding the pathophysiology underlying critical diseases such as dysmenorrhea, adenomyosis, impaired sperm and oocyte transport, embryo implantation failure, or repeated miscarriage. 2 , 3 , 5 , 6 , 13 , 14 These insights could ultimately lead to more targeted diagnostic and therapeutic strategies in reproductive medicine. 5 , 17 The MF phase was distinguished by the highest contractile energy (amplitude and contractility index), lowest contraction frequency and local coordination, especially in the fundus. The present findings are consistent with preliminary results concerning the amplitude and frequency of fundal contractions, as assessed by IC‐EHG. 17 This intense activity of lower frequency during this phase could be related to the proximity of menstruation, when contractions are of maximum amplitude with a low occurrence rate, 2 , 28 reflecting a transient physiological process towards the periovulatory phase. To generate these strong contractions, it is necessary to recruit a greater number of myocytes, which in turn hinders coordination among them, as evidenced by the reduced coordination index. Regarding uterine regional differences, it is plausible that contractions are more powerful in the fundus than in the lower segment, potentially aiding effective uterine emptying, although this hypothesis requires further investigation. Moreover, the fundus has been shown to exhibit a higher density of oxytocin receptors 9 , 29 and of myocytes, 30 which may contribute to its enhanced contractile strength, greater basal tone and longer dwell time in the contractile state. In addition, the fundus exhibited consistently lower coordination throughout the menstrual cycle in comparison to the lower uterine segment. This phenomenon may be attributable to regional variations in the expression of gap junctions, which play a pivotal role in facilitating cell‐to‐cell communication and coordinating their activity. In rat uterine horns, the expression of connexin 43(Cx‐43)—the most relevant protein in the formation of gap junctions 31 , 32 —has been reported to be more abundant at the cervical end than at the ovarian end 33 aligning with our findings. However, regional differences in Cx43 expression, have not been directly studied or established in the non‐pregnant junctional zone, as reported by. 34 Although limited, existing studies on estrogen receptor expression, which promotes Cx43 synthesis, suggest no regional variation, 35 indirectly supporting the assumption of uniform Cx43 distribution. Therefore, the lower coordination observed within the fundus may be attributable to the elevated myocyte density present within this region. 30 This heightened density has the potential to introduce complexity into cell–cell signaling processes, thereby impeding synchronization. The disparities obtained in the peristalsis features between the fundus and the lower segment highlight a significant regional specialization in the contractile behavior of the uterus, as suggested by other authors. 11 Occurring during the periovulatory period, the EL phase was characterized by the highest contraction frequency and maximum local coordination observed throughout the cycle. This aligns with previous reports describing a periovulatory peak in uterine contractile activity. 6 , 13 , 29 Electrophysiologically, this pattern is consistent with increased myometrial excitability driven by elevated estradiol concentrations. 29 , 31 Acting via estrogen receptor alpha (ERα), estradiol modulates the expression and function of ion channels, 35 thereby enhancing depolarization and propagation of electrical activity across the myometrium. 27 , 31 , 36 Furthermore, ERα stimulation has been shown to upregulate the expression of Cx43, and hence of gap junctions accounting for the promoted intercellular coupling and electrical synchrony in this menstrual phase. Consistent with this mechanism, basal amplitude was elevated in both uterine regions during EL, likely reflecting increased membrane excitability and contractile readiness. Furthermore, during this phase, the uterus is in its contracted state for most of the time. This is observed not only in the fundus but also in the lower segment, thereby reinforcing the profile of sustained, synchronized activity during this stage. Consequently, the EL phase is defined by a well‐coordinated and frequent but energetically moderate contractile pattern. Such an organized peristaltic environment may collectively support reproductive functions such as sperm transport and endometrial receptivity in the early post‐ovulatory window. 11 It is widely accepted within the field of reproductive physiology that uterine quiescence is an indispensable component for successful embryo implantation, which typically occurs during the LL phase. 3 , 6 , 8 Previous studies using intrauterine pressure recordings and TVUS have reported a reduction in contraction frequency during this phase. 7 , 37 Whilst a similar frequency reduction was also observed in comparison to the EL phase, the present findings suggest that this quiescent state is more accurately characterized by a significant reduction in contraction intensity, its temporal predominance as well as minimum basal tone, rather than contraction frequency alone. Notably, IC‐EHG recorded twice the contraction frequency during the LL phase compared to TVUS, 2 consistent with a preceding pilot study, 17 likely reflecting its superior sensitivity to low‐amplitude contractions that may be missed by conventional imaging methods. In addition, this minimal peristaltic activity in terms of contraction amplitude, energy, percentage of time and basal tone, was observed in both the bottom and lower segments, supporting the notion of global quiescence during this phase. This reduced contractile activity is consistent with the endocrine environment of the LL phase, 8 , 38 dominated by high progesterone levels. 12 , 29 Progesterone modulates ion channel expression and function, particularly by enhancing potassium conductance and reducing calcium influx, stabilizing membrane potential and prolonging the refractory period. 36 , 38 In addition, the reduction of estradiol concentration in the LL phase diminishes that of Cx43, thus leading to the observed reduction of the local coordination. In summary, the LL phase is therefore a phase characterized not so much by a minimum contractile frequency as by spending the least amount of time in a contractile state, with minimal intensity, energy, and basal tone. This electrophysiological environment is consistent with the requirements for successful embryo implantation. This study introduces IC‐EHG as a powerful electrophysiological tool for the objective and comprehensive evaluation of uterine peristalsis across menstrual phases and anatomical regions. Unlike conventional non‐pregnant uterine assessment techniques, which measure the indirect mechanical effects of myocyte contractions, that is, pressure and deformation, IC‐EHG enables the direct capture of the myoelectrical activity with high temporal and spatial resolution. This allows for the detection of subtle, low‐amplitude or transient contractions that may otherwise remain undetected. 2 , 15 One of the key contributions of IC‐EHG is its capacity to quantify local coordination of the electrical activity. In this study, distinct spatiotemporal patterns throughout the menstrual cycle were revealed, underlining its potential as a biomarker of myocyte synchronization. This feature may prove clinically valuable in identifying conditions characterized by dysperistalsis or impaired contractile integration, such as adenomyosis, endometriosis, or unexplained infertility. 5 , 6 , 13 , 14 Furthermore, the IC‐EHG facilitated the identification of phase‐specific contractile patterns, extending beyond the mere frequency of contraction, encompassing parameters such as contraction amplitude, basal amplitude, energy, contraction time percentage, and local coordination. This comprehensive profiling offers a clinically relevant framework for optimizing fertility‐related interventions, as it establishes reference values in healthy women that can serve as a benchmark for detecting pathological alterations. For instance, the objective and quantitative characterization of uterine peristalsis, employing techniques such as IC‐EHG, may reveal abnormally low frequency and/or amplitude activity during the periovulatory period, or dyskinetic patterns (i.e., abnormal activity that could include alterations in the direction or coordination of contractions). Such alterations could impede the rapid and targeted transport of sperm to the oocyte, thereby affecting fertilization. Such uterine peristaltic dysfunction would represent a potential cause of infertility, suggesting the need for further evaluation to consider modulation of uterine activity (e.g. through pharmacological interventions) or the application of assisted reproductive techniques, such as in vitro fertilization, to optimize the chances of conception. Similarly, excessive or disorganized activity during the mid‐ or late luteal phase could underlie implantation failure. Consequently, the identification of these conditions through the IC‐EHG technique could assist clinicians in making informed decisions regarding the utilization of uterine activity modulation agents or the postponement of embryo transfer in the context of ART. Beyond assisted reproduction, IC‐EHG may also aid in the diagnosis of gynecological disorders associated with abnormal peristalsis, including endometriosis and adenomyosis. Thus, electrophysiological assessment could complement hormonal markers and imaging, providing an objective tool to support individualized decision‐making in reproductive medicine. The ability of IC‐EHG to detect region‐specific alterations, such as the intense but disorganized fundal contractility observed in the MF phase, may also aid in the early diagnosis of contractile dysfunction. These regional findings offer new perspectives for the development of targeted therapies aimed at restoring physiological peristalsis and improving reproductive outcomes. Finally, the capacity of IC‐EHG to provide objective quantitative information renders it optimally suited to the longitudinal monitoring of the effects of hormonal therapies or surgical interventions on uterine peristalsis and electrophysiological condition. The present study is subject to several limitations. First, the relatively small sample size of 40 participants may limit the generalizability of the findings. Future studies involving larger and more diverse cohorts are needed to validate and extend these results. Additionally, future research should address both intra‐session and inter‐cycle variability, reflecting fluctuations within a single recording or across menstrual cycles respectively. Understanding these dynamic phenomena is essential for the comprehensive characterization of the electrophysiological behavior of the uterus. Second, the exclusion of participants with common gynecological conditions such as fibroids, endometriosis, or adenomyosis limits the applicability of these findings to broader clinical populations. Nevertheless, the results obtained here may serve as reference values for fertile healthy women, providing a baseline against which pathological uterine contractility patterns can be compared. In this context, further studies are needed to investigate the correlation between IC‐EHG‐derived peristaltic features and reproductive outcomes, such as implantation and pregnancy rates, to determine the clinical relevance of these electrophysiological biomarkers. Finally, although the intrauterine catheter used for IC‐EHG recording could theoretically stimulate or alter myometrial activity, the consistent and phase‐dependent peristaltic patterns observed across the cohort suggest that the measurements primarily reflect the uterus's intrinsic electrophysiological behavior. This interpretation is in line with previous findings by van Gestel, 39 who reported minimal artefactual influence from catheter placement on uterine activity. Additionally, potential conflicts of interest should be considered when interpreting the findings, as several authors are employees or shareholders of the company producing the IC‐EHG device. To ensure independence and minimize bias, all data were collected by independent clinicians who are co‐authors of this work, and analysis was performed in a blinded manner using predefined, objective algorithms. Peristaltic activity features were computed systematically through objective mathematical formulas, rather than relying on human interpretation as in other techniques. The discussion is based on comparisons with pre‐established literature and current knowledge in the field.

This study provides the first objective characterization of uterine contractility across menstrual phases and anatomical regions using IC‐EHG. The findings reveal distinct spatiotemporal patterns of myometrial activity throughout the cycle, offering new insights into the physiological modulation of uterine function. The MF phase was characterized by the highest contractile energy in the fundus, driven by maximal contraction amplitude despite relatively low frequency and coordination. This indicated strong yet disorganized activity. As the cycle progressed, a progressive decline in fundal contractile output was also observed. Conversely, the EL phase, occurring in the periovulatory window, was marked by the highest contraction frequency, optimal local coordination, and elevated basal amplitude. Notably, the uterus is in its contracted state for a longer period of time than during any other phase. This indicates a sustained and coordinated contractile state of the uterus that is favorable to gamete transport and endometrial receptivity. It has been confirmed that the LL phase is the most quiescent phase of the menstrual cycle, a state which facilitates embryo implantation; however, this quiescence is more clearly evident through decreased contractile amplitude, energy, temporal predominance, and basal tone, rather than a decrease in frequency. The fundus is posited to be a more active uterine region than the lower segment, especially during the MF phase, with a longer contractile state and greater amplitude, although with poorer local coordination. These spatial patterns support functional specialization along the uterine axis. IC‐EHG has been demonstrated to outperform conventional imaging techniques (TVUS, MRI) in detecting higher contraction frequencies, as well as quantifying additional peristalsis features in a direct, objective and easily reproducible manner. This establishes a novel paradigm for helping clinicians in diagnosing dysperistalsis, personalizing drug dosages, guiding fertility interventions, and ultimately improving reproductive outcomes.

The non‐gravid uterus is an active smooth muscle organ 1 that, throughout the menstrual cycle, exhibits distinct peristaltic patterns essential for sperm and oocyte transport, embryo implantation, and menstrual shedding. 2 , 3 This wave‐like activity originates in the subendometrial myometrial layer, also known as the junctional zone, and results from the electrical activity of myometrial smooth muscle cells. 4 , 5 , 6 , 7 It is characterized by spike action potentials generated by voltage‐ and time‐dependent changes in membrane ionic permeability. 4 A complex system of local and systemic endocrine signaling, including primarily the ovarian hormones (estrogen and progesterone), 1 , 6 , 8 as well as oxytocin and prostaglandins, 9 , 10 , 11 modulates uterine electrophysiology by altering the resting membrane potential and the electrical activity of myometrial cells. 2 This hormonal regulation orchestrates changes in the frequency, amplitude, and direction of uterine peristalsis throughout the different phases of the menstrual cycle, thereby optimizing the uterus's function for reproduction. 3 , 5 , 8 , 11 , 12 , 13 The evaluation of uterine peristalsis has traditionally relied on using transvaginal ultrasound (TVUS) and magnetic resonance imaging (MRI). 3 , 14 Although TVUS is widely accessible, its imaging quality is highly dependent on the transducer's position and orientation, 2 , 13 , 15 leading to significant inter‐observer variability. 16 In contrast, while MRI provides detailed images, it is both time‐consuming and expensive. 2 , 13 , 14 Furthermore, uterine peristalsis is typically assessed based on the frequency of contractions, as determined through visual and subjective post‐recording observation of fast‐forwarded videos of short duration (3–5 min). Consequently, there is a lack of methods that offer more objective and comprehensive information for monitoring and evaluating uterine peristalsis and its underlying electrophysiological dynamics in clinical practice. Electrohysterography (EHG) offers a promising alternative method for monitoring uterine peristalsis. 17 It records the myoelectrical activity of the uterus resulting from the coordinated depolarization and repolarization of billions of myometrial cells. 4 , 18 , 19 The resulting signals include spike bursts associated with uterine contractions (CTs) as well as basal activity during resting periods. 4 , 18 , 20 The direct relationship between electrical and mechanical activity of the uterus during pregnancy has been extensively documented. 18 , 19 , 21 A recent pilot study involving 15 women has demonstrated the potential of intracavitary electrohysterography (IC‐EHG) in characterizing fundamental aspects of peristalsis in the uterine fundus throughout the menstrual cycle. 17 Building upon this previous pilot study, which established the feasibility of IC‐EHG in a small cohort with a limited focus on fundal contraction frequency and amplitude, the present work expands the sample size substantially and broadens the analysis. Specifically, the characterization of peristalsis is extended to other uterine regions and additional relevant aspects of contractile activity, such as basal tone, percentage of time in contraction, local coordination, and an overall contractility index are evaluated. Together, these advances provide a more comprehensive understanding of the spatial and temporal dynamics of uterine activity and their potential role in reproductive function and embryo implantation.

Sonda Devices S.L. is a company owned by IVIRMA. JAR is an employee and shareholder of Sonda Devices S.L. JMMT and ADM are salaried employees of Sonda Devices S.L. JGC maintains a consulting and advisory relationship with Sonda Devices S.L. and is also a shareholder. All other authors have no conflicts of interest to declare.

A cohort study with a prospective, multicenter design was carried out across IVI clinics in Madrid, Barcelona, and Valencia (Spain). Between February 2024 and September 2024, the study aimed to recruit a total of 50 participants. However, due to time constraints, only 40 patients were ultimately enrolled. Each patient was included in the study only once to ensure data consistency and avoid any potential biases. All enrolled patients provided written informed consent. The inclusion criteria for the study were as follows: (i) women aged between 18 and 39 years, (ii) proven fertility (i.e. observation of intrauterine gestational sac with a heartbeat 3 weeks after the positive human chorionic gonadotrophin test), (iii) regular menstrual cycles (21–35 days), and (iv) a body mass index (BMI) of 18.5–27.0 kg/m 2 . Exclusion criteria included the following: (i) pregnancy, (ii) uterine abnormalities (e.g., fibroids, adenomyosis), (iii) recent hysteroscopy, (iv) use of hormonal contraceptives or medications affecting uterine contractility, and (v) gynecological conditions (e.g., irritable bladder syndrome, tubal pathology, ectopic pregnancy, or severe dysmenorrhea). The study comprised four visits per patient: a screening visit and three visits during the subsequent menstrual cycle, during which uterine peristalsis was assessed. During the screening visit, demographic and hormonal characteristics were collected, including age, BMI, number of gestations, deliveries, miscarriages, and ectopic pregnancies, as well as serum progesterone and estradiol levels. Luteinizing hormone (LH) test strips were provided to the patients for home testing. The three additional visits were conducted during the following phases of the menstrual cycle: mid‐follicular (MF, 6–8 days after menstruation onset), early luteal (EL, 2–4 days after LH+), and late luteal (LL, 7–9 days after LH+). During each visit, IC‐EHG recordings and endometrial thickness measurements were performed, with the corresponding cycle day also annotated. Additionally, uterine internal cervical os‐to‐fundus distance and position were assessed during the first IC‐EHG recording visit. To perform the IC‐EHG recordings, the Sonda Devices N01 (Sonda Devices S.L., Valencia, Spain), the first medical device designed specifically for this purpose, 22 was employed. This system consists of a medical device for signal conditioning, monitoring, and acquisition, along with a disposable multipolar electrophysiological catheter for detecting myometrial activity. 17 The system records signals within a bandwidth of 0.1–4 Hz 19 , 23 , 24 at a sampling rate of 500 Hz. The catheter is inserted into the uterine cavity under abdominal ultrasound guidance, with the tip placed next to the uterine fundus, while the remaining poles are distributed along the uterine wall towards the cervix. 17 Supporting information includes a Video  S1 illustrating the catheter positioning under ultrasound control. A ground electrode is positioned on the patient's right buttock. Each electromyographic recording session lasted 30 min. All IC‐EHG recordings underwent a quality check, and those with poor electrophysiological catheter contact or suboptimal electrode placement were excluded from the study. Figure  1 presents a flowchart illustrating the inclusion of patients and the retention of records. Participants and recording inclusion flow chart. Following standard methods for analyzing electrohysterogram recordings, segmentation was performed to identify the onset and offset of each contraction (CT) in IC‐EHG recordings, as well as to exclude non‐physiological segments such as motion artifacts. From each IC‐EHG recording, the following features were extracted to characterize peristaltic physiological patterns across menstrual phases and uterine regions: Frequency of contractions measures the rhythmicity of the CTs and is expressed as CTs per minute. Amplitude of contractions reflects the strength of myometrial contractions. It is calculated as the root mean square of the squared voltage values during the CT and is expressed in microvolts (μV). Amplitude of basal activity assesses the signal intensity of non‐contractile periods which can be indicative of the basal tone. It is calculated as the root mean square of the squared voltage values during the resting period and is expressed in microvolts (μV). Contraction time percentage quantifies the temporal dominance of contractile activity within each peristaltic cycle. A peristaltic cycle is defined as the interval between the offset of one contraction and the offset of the next, encompassing both the contraction itself and the preceding basal period. This feature is calculated as the ratio between the duration of the contraction and that of the entire cycle and is expressed as a percentage (%). Contractility index assesses the total contractile energy exerted per unit of time. It is calculated by dividing the sum of the energy (amplitude × duration) of the contractions by the recording time considered. It is expressed in microvolt‐seconds per minute (μV·s/min). Local coordination index estimates the degree of regularity or predictability in the uterine myoelectrical signal. Higher values indicate more regular and coordinated activity, while lower values suggest greater signal complexity or discoordination. It is calculated as the inverse of the Sample Entropy of the signal. The latter is a widely used metric for assessing the regularity of bioelectrical signals in general 25 and of pregnancy electrohysterographic signals in particular. 20 It is expressed in arbitrary units. Frequency of contractions measures the rhythmicity of the CTs and is expressed as CTs per minute. Amplitude of contractions reflects the strength of myometrial contractions. It is calculated as the root mean square of the squared voltage values during the CT and is expressed in microvolts (μV). Amplitude of basal activity assesses the signal intensity of non‐contractile periods which can be indicative of the basal tone. It is calculated as the root mean square of the squared voltage values during the resting period and is expressed in microvolts (μV). Contraction time percentage quantifies the temporal dominance of contractile activity within each peristaltic cycle. A peristaltic cycle is defined as the interval between the offset of one contraction and the offset of the next, encompassing both the contraction itself and the preceding basal period. This feature is calculated as the ratio between the duration of the contraction and that of the entire cycle and is expressed as a percentage (%). Contractility index assesses the total contractile energy exerted per unit of time. It is calculated by dividing the sum of the energy (amplitude × duration) of the contractions by the recording time considered. It is expressed in microvolt‐seconds per minute (μV·s/min). Local coordination index estimates the degree of regularity or predictability in the uterine myoelectrical signal. Higher values indicate more regular and coordinated activity, while lower values suggest greater signal complexity or discoordination. It is calculated as the inverse of the Sample Entropy of the signal. The latter is a widely used metric for assessing the regularity of bioelectrical signals in general 25 and of pregnancy electrohysterographic signals in particular. 20 It is expressed in arbitrary units. All the features were computed using Matlab (Mathworks, Natick, USA). Given the novelty of IC‐EHG outputs, there were no prior effect‐size estimates to calculate the necessary sample size. Instead, the target enrolment was based on analogous uterine‐peristalsis investigations. Sammali et al. examined transabdominal EHG in 11 non‐pregnant volunteers, 26 Wang et al. mapped 4D electrical activation in 26 women, 13 and Rees et al. tracked contractile dynamics via 2D TVUS speckle‐tracking in 4–26 subjects depending on the menstrual cycle phase. 15 The sample of the present study meets or exceeds these foundational cohorts, providing robust exploratory data and informing future hypothesis‐driven trials. With a sample size of 40 participants per condition in a paired study, a significance level of 0.05, and a statistical power of 0.8, the minimum detectable effect size (Cohen's d) is approximately d ≈ 0.454, which can be deemed a moderate effect. Normality of all continuous variables was assessed by the Shapiro–Wilk test. Parameters that were not normally distributed are reported as medians with their interquartile ranges (IQR), whereas normally distributed data are presented as means and standard deviations (SD). Uterine peristalsis features were computed for each peristaltic event. For each recording session, the median value of each IC‐EHG feature was designated as the representative value. The Wilcoxon signed‐rank test was employed to facilitate a comprehensive and consistent framework for the IC‐EHG feature comparison between uterine regions and menstrual phases. This approach was adopted to circumvent the inconsistencies that might be introduced by employing a combination of mixed testing strategies, given that certain distributions aligned with the normality hypothesis, while others did not. In all cases, a p ‐value <0.05 was considered to indicate statistical significance, and a 95% confidence interval (CI) of the mean difference was used to assess the precision of the estimates. The effect size (Cohen's ‘d’) was also calculated for each paired comparison. Correction for multiple comparisons was not applied, as the primary aim was to identify potential differences and trends that warrant further investigation in larger, hypothesis‐driven studies. Statistical analyses were performed using Matlab (Mathworks, Natick, USA).

Video S1. Intracavitary electrohysterography catheter placement under abdominal ultrasound guidance. Table S1. Median and quartile (Q1 and Q3) of uterine activity frequency (contractions/min), contraction and basal amplitude (μV), contraction time (%), contractility index (μV·s/min) and local coordination (a.u.) in fundus and lower segment of the uterus and different phases of the menstrual cycle (EL, early luteal; LL, late luteal; MF, mid‐follicular). Table S2 . Comparison of uterine activity parameters across menstrual cycle phases (EL, early luteal; LL, late luteal; MF, mid‐follicular) for both uterine regions (fundus and lower segment). Table S3 . Comparison of uterine activity parameters between uterine regions (fundus vs. lower segment) within each menstrual cycle phase (EL, early luteal; LL, late luteal; MF, mid‐follicular).