# Intrapartum monitoring of the fetal heart rate using transabdominal electrocardiography: a reliability and accuracy study.

Anne Wojtanowski, Charles Garabedian, Pierre Charlier, Pierre Prot, Louise Ghesquiere, Julien de Jonckheere

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Anne Wojtanowski, Charles Garabedian, Pierre Charlier, Pierre Prot, Louise Ghesquiere, et al. In-trapartum monitoring of the fetal heart rate using transabdominal electrocardiography: a reliability and accuracy study. Journal of Gynecology Obstetrics and Human Reproduction, 2025, Journal of Gynecology Obstetrics and Human Reproduction, 54 (5), pp.102942. 10.1016/j.jogoh.2025.102942. hal-05076891

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Original Article

# Intrapartum monitoring of the fetal heart rate using transabdominal electrocardiography: A reliability and accuracy study

Anne Wojtanowski<sup>a,b,*</sup>, Charles Garabedian<sup>b,c</sup>, Pierre Charlier<sup>a,b</sup>, Pierre Prot<sup>d</sup>,
Louise Ghesquiere<sup>b,c</sup>, Julien De jonckheere<sup>a,b</sup>

<figure><img src="image_4.png" /></figure>

ARTICLE INFO

Keywords:

Fetal heart rate monitoring

Non-invasive fetal electrocardiography

Signal quality

A B S T R A C T

**Introduction:** In this study, we evaluated the performance of an intrapartum monitoring system (the TOCONAUTE) for fetal heart rate estimation using transabdominal electrocardiography.

**Method:** This prospective study was conducted in Lille University Hospital's maternity ward between August 2022 and November 2023 to assess the performance of the TOCONAUTE compared to a cardiotocograph (CTG). Pregnant women in labor at 36 weeks or more of gestation with a singleton in cephalic presentation were eligible for enrollment in the study. In addition to the cardiotocograph, the participants received an abdominal electrode patch after providing their written informed consent. The positive percent agreement with the cardiotocograph indicated reliability, and the Bland-Altman analysis determined accuracy.

**Results:** Of the 60 patient recruited, 35 patients were included in the recording analysis. The recordings had a mean duration of 519 ± 210 min. The TOCONAUTE's fetal heart rate (FHR) overall positive percent of agreement (PPA) was 84.2 ± 10.4 %. The PPA was greater than 80 % for all labor phases, except for expulsion (44.7 %). The overall mean bias was 1.7 ± 2.4. For the FHR, the mean bias was lower than 2 bpm for all labor phases, except for expulsion (14.13 ± 15.04 bpm).

**Conclusion:** Compared with traditional CTG, this study demonstrated the TOCONAUTE's reliability and accuracy for FHR estimation in all labor phases except expulsion.

Introduction

During labor, cardiotocography (CTG) is the gold standard for evaluating fetal well-being through analysis of the fetal heart rate (FHR). However, it presents a high rate of inter-observer variability [1,2] and low specificity to predict poor neonatal acidosis and outcomes [3,4]. In cases of non-reassuring FHR tracing, second-line methods can be used, such as fetal blood sampling and fetal ECG ST segment analysis. These methods are also discussed in the literature [5-8].

Because of the well-known limits of the technologies cited above and the invasiveness of second-line tools, a more sensitive continuous and noninvasive method of fetal monitoring is needed. Analysis of the autonomic nervous system (ANS) activity could constitute an efficient fetal homeostasis monitoring method. Indeed, under hypoxia, the fetus develops several coping mechanisms, including increased O₂ extraction

by tissues and redistribution of blood flow towards the vital organs. These adaptive mechanisms are regulated by the ANS via sympathetic and parasympathetic activities. The ANS is highly sensitive to hypoxia and acidosis through various well-described mechanisms, including the stimulation of chemoreceptors and baroreceptors and the direct actions of hypoxia and hypercapnia on the brain stem [9,10].

Heart rate and heart rate variability (HRV) are directly controlled by the ANS, and previous studies have suggested that fetal HRV is affected by metabolic acidosis during labor [11]. Several HRV analysis methods have been evaluated in fetal experimental models, with interesting results regarding fetal acidosis (or fetal severe acidosis) or inflammation prediction [12-17]. However, these HRV analysis techniques need to overcome several technical barriers before they can be applied to human fetuses. Indeed, the CTG used in clinical practice cannot obtain a beat-to-beat FHR, which constitutes an important limitation for HRV

<footnote><sup>a</sup> CHU Lille, CIC-IT 1403, F-59000 Lille, France</footnote>

<footnote><sup>b</sup> Univ. Lille, ULR 2694 - METRICS, F-59000 Lille, France</footnote>

<footnote><sup>c</sup> CHU Lille, Department of Obstetrics, F-59000 Lille, France</footnote>

<footnote><sup>d</sup> Bioserenity, F-75013 Paris, France</footnote>

<footnote>* Corresponding author at: CHU Lille, CIC-IT 1403, F-59000 Lille, France.</footnote>

<footnote>E-mail address: anne.wojtanowski@chu-lille.fr (A. Wojtanowski).</footnote>
analysis in human fetuses. In 2018, the CIC-IT 1403 (CHU Lille), Lille University, and the BIOSERENITY company obtained a grant from the French National Research Agency (ANR) to overcome these technological barriers to investigating HRV analysis in human fetuses (ANR 2018-CE19-PrevAP project). Ending in October 2022, this project led to the development of a medical device (TOCONAUTE) allowing beat-to-beat FHR assessment through transabdominal (i.e., noninvasive) fetal ECG acquisition.

Thus, the objective of this study was to assess the performance of the
TOCONAUTE compared to the gold standard FHR technology (i.e.,
CTG).

Materials and methods

This prospective study was conducted in Lille University Hospital's maternity ward to assess the performance of the TOCONAUTE compared to the gold standard FHR technology: CTG (Philips Avalon, Philips Healthcare). Patients were recruited between August 2022 and November 2023.

Pregnant women in labor at 36 weeks or more of gestation with a singleton in cephalic presentation were eligible for enrollment in the study. Pregnant women who were hospitalized for a medical termination of pregnancy or death in utero were not included. All patients were initially monitored using the CTG Doppler ultrasound (DU) technique, and only CTG was used for clinical decisions. The CTG sensor positions were adjusted by midwives when clinically required.

Data collection and processing

The FHR and maternal heart rate (MHR) were recorded using a
Philips Avalon device's (Philips Healthcare, Andover, MA, USA) nu-
merical output.

The TOCONAUTE consists of a seven-electrode patch (six leads) linked to an electronic module for signal amplification, filtering, digitalization (1000 Hz, 32 bits), and transmission via Bluetooth low energy (Fig. 1A). Raw transabdominal signals are retrieved by signal processing software installed on a personal computer (PC) for FHR and MHR computation and visualization (Fig. 1B).

After obtaining written consent from the patient, a clinical research
assistant set up the TOCONAUTE to record the FHR and MHR. Before the
TOCONAUTE's electrode positioning, the skin was prepared using
medical abrasive paper to improve the transabdominal signal quality.
The electrode patch was then positioned on the abdomen and connected
to the electronic module, and the TOCONAUTE software began
recording.

The TOCONAUTE and CTG recordings were stopped after childbirth,
departure for the operating room for a cesarean section, or when the
recording duration exceeded 12 h, which corresponds to the TOCO-
NAUTE's battery life.

CTG used in clinical practice does not allow for obtaining beat-to-beat FHR, and FHR values provided by the CTG are the average of 2 or 3 beats that are resampled at 4 Hz. To be synchronized with CTG to allow sample-to-sample comparison, the TOCONAUTE's FHR was therefore averaged on 3 beats and resampled. Both the averaged FHRs and MHRs were sampled at 4 Hz.

Outcome measures

The primary outcome of the study was to assess the TOCONAUTE's performance in measuring the FHR by using CTG as the gold standard. CTG reports three levels of signal quality: bad, medium, and good. The CTG output was considered valid if the FHR was present and the signal quality was good or medium. We also computed the CTG success rate as the percentage of time the CTG provided an FHR value with a good or medium signal quality. The reliability of the TOCONAUTE values was assessed only on valid CTG values, following the method described by Cohen et al. [18]. For each sample, the TOCONAUTE FHR was considered valid if it was within ± 10 % of the CTG FHR value.

Statistical analysis

For each patient, we assessed the reliability of the measurement by
computing the positive percent of agreement (PPA), which represents
the percentage of time the TOCONAUTE generated an FHR value within
±10 % of the gold standard value.

We also assessed the accuracy of the measures using a Bland-Altman analysis. This test allows for computing the bias (mean standard error) as well as the lower and upper limits of agreement. A Bland-Altman test was performed for each subject individually. The mean bias, as well as a 95 % confidence interval (CI) and lower and upper limits of agreement, were first determined per participant and subsequently calculated as the mean and standard deviation of all participants.

This analysis was performed both for the overall recording duration
and independently for each labor phase. The latent phase is defined as a
cervix dilatation between 2 and 5 cm. The active phase is defined as a
cervix dilated up to 5 cm. The descent phase is defined as a cervix fully
dilated until the beginning of pushing efforts. The expulsion phase be-
gins with pushing efforts and ends with birth. The first stage comprises
the latent and active phases, and the second stage comprises the descent
and expulsion phases.

The same analyses were performed to assess the MHR signal's reli-
ability and accuracy.

The categorical variables were expressed as frequency and percent-
age, and the quantitative variables were expressed as mean ± standard
deviation. To assess the primary and secondary outcomes, both signals
were compared sample by sample using a Bland–Altman test. The data
were analyzed using RStudio (RStudio version 2024.04.2 + 764) [19].

<figure><img src="image_5.png" /><figcaption>Fig. 1. A. TOCONAUTE and CTG placed on the mother's abdomen. Fig. 1B. Image of the TOCONAUTE's FHR and MHR recording.</figcaption></figure>
## Ethics

This study was approved by the Comité de Protection des Personnes Est IV on May 18, 2022 (n°2022-A00107-36) and by the Agence Nationale de Sécurité du Médicament et des produits de santé on May 16, 2022. The study was registered on ClinicalTrials.gov (NCT05263427) before recruitment started.

## Results

### Participants

Sixty patients were recruited during the inclusion period. The first 10 patients were excluded due to a data export software problem that prevented TOCONAUTE data analysis. After a software update to correct this bug, the problem no longer appeared in the following recordings. Two patients were excluded for non-recorded TOCONAUTE files and two others for non-recorded CTG files. Of the 46 remaining records, 10 were excluded due to a bad TOCONAUTE signal quality, which rendered synchronization with the CTG impossible. One record was excluded for MHR/FHR rhythm inversion.

The included patients had a mean age of 32 ± 4 years. Their mean BMI before pregnancy was 25.5 ± 6.0, their mean gestational age was 39.8 ± 0.98 weeks, 17 % of them had a cesarean, 74 % had a vaginal birth, and 9 % had an operative vaginal delivery (Table 1).

The TOCONAUTE and CTG recordings had a mean duration of 519 ± 210 min. The mean duration of the latent phase recordings was 380 ± 286 min, and the mean duration of the active phase recordings was 130 ± 90 min. The mean duration of the descent phase recordings was 80 ± 58 min, and the mean duration of the expulsion phase recordings was 11 ± 10 min. The FHR CTG success rate for all the recordings was 94 ± 3 %, and the FHR CTG success rates for each phase were 94 ± 3 % (latent), 93 ± 5 % (active), 94 ± 3 % (descent), and 62 ± 22 % (expulsion) (Table 2).

The newborns had a mean weight of 3657 g ± 510 g and a mean height of 49.7 cm ± 2.6 cm, and 5.7 % of them had to be transferred to the neonatal intensive care unit (Table 1).

<table><caption>**Table 1**
Maternal and newborn characteristics.</caption><thead><tr><th></th><th>ITT n = 60</th><th>Per protocol n = 35</th></tr></thead><tbody><tr><td><strong>Maternal characteristics</strong></td><td></td><td></td></tr><tr><td>Age, years</td><td>31 ± 5</td><td>32 ± 4</td></tr><tr><td>Weight before pregnancy, kg</td><td>71 ± 17</td><td>70 ± 18</td></tr><tr><td>BMI before pregnancy, kg/m²</td><td>26.1 ± 6.3</td><td>25.5 ± 6.0</td></tr><tr><td>Gestational age, weeks</td><td>39.9 ± 1.0</td><td>39.8 ± 0.98</td></tr><tr><td>Diabetes</td><td>11 (18 %)</td><td>7 (20 %)</td></tr><tr><td>Pre-eclampsia</td><td>0 (0 %)</td><td>0 (0 %)</td></tr><tr><td>Oxytocin</td><td>41 (68 %)</td><td>26 (74 %)</td></tr><tr><td>Meconial liquid</td><td>5 (8.3 %)</td><td>3 (8.6 %)</td></tr><tr><td>Manual rotation</td><td>3 (5 %)</td><td>2 (5.7 %)</td></tr><tr><td>Number of maternal position changes</td><td>7 ± 3</td><td>8 ± 3</td></tr><tr><td>Epidural</td><td>60 (100 %)</td><td>35 (100 %)</td></tr><tr><td>Birth</td><td></td><td></td></tr><tr><td>Cesarean</td><td>9 (15 %)</td><td>6 (17 %)</td></tr><tr><td>Instrumental birth</td><td>9 (15 %)</td><td>3 (9 %)</td></tr><tr><td>Natural birth</td><td>42 (70 %)</td><td>26 (74 %)</td></tr><tr><td><strong>Newborns' characteristics</strong></td><td></td><td></td></tr><tr><td>Sex, female</td><td>26 (43 %)</td><td>15 (43 %)</td></tr><tr><td>Weight, g (n = 59)</td><td>3588 ± 461</td><td>3657 ± 510</td></tr><tr><td>Height, cm (n = 57)</td><td>49.4 ± 3.4</td><td>49.7 ± 2.6</td></tr><tr><td>APGAR 1 min &lt; 7</td><td>4 (7 %)</td><td>1 (3 %)</td></tr><tr><td>APGAR 5 min &lt; 7</td><td>3 (5 %)</td><td>1 (3 %)</td></tr><tr><td>APGAR 10 min &lt; 7 (n = 59)</td><td>2 (3 %)</td><td>1 (3 %)</td></tr><tr><td>pH (n = 59)</td><td>7.20 ± 0.08</td><td>7.20 ± 0.08</td></tr><tr><td>Lactates (n = 56)</td><td>4.99 ± 2.11</td><td>5.05 ± 2.08</td></tr><tr><td>NICU transfer</td><td>4 (6.7 %)</td><td>2 (5.7 %)</td></tr></tbody></table>

The results are expressed as mean ± standard deviation or frequency (percentage). ITT: intention to treat; APGAR score: appearance, pulse, grimace, activity, and respiration score; NICU: neonatal intensive care unit.

<table><caption>**Table 2**
Recording characteristics.</caption><thead><tr><th>Recording characteristics</th><th>ITT n = 60</th><th>Per protocol n = 35</th></tr></thead><tbody><tr><td><strong>Overall</strong></td><td></td><td></td></tr><tr><td>Recording duration, min</td><td>472 ± 211</td><td>519 ± 210</td></tr><tr><td>FHR CTG signal quality %</td><td>93 ± 4</td><td>94 ± 3</td></tr><tr><td>MHR CTG signal quality %</td><td>81 ± 12</td><td>82 ± 12</td></tr><tr><td><strong>Stage 1</strong></td><td></td><td></td></tr><tr><td>Stage duration, min</td><td>418 ± 241</td><td>455 ± 263</td></tr><tr><td>FHR CTG signal quality %</td><td>93 ± 6</td><td>93 ± 6</td></tr><tr><td><strong>Latent</strong></td><td></td><td></td></tr><tr><td>Phase duration, min</td><td>354 ± 258 n = 52</td><td>380 ± 286</td></tr><tr><td>FHR CTG signal quality %</td><td>94 ± 4</td><td>94 ± 3</td></tr><tr><td><strong>Active</strong></td><td></td><td></td></tr><tr><td>Phase duration, min</td><td>126 ± 84 n = 43</td><td>130 ± 90</td></tr><tr><td>FHR CTG signal quality %</td><td>93 ± 6</td><td>93 ± 5</td></tr><tr><td><strong>Stage 2</strong></td><td></td><td></td></tr><tr><td>Stage duration, min</td><td>88 ± 70</td><td>87 ± 61</td></tr><tr><td>FHR CTG signal quality %</td><td>88 ± 10</td><td>90 ± 5</td></tr><tr><td><strong>Descent</strong></td><td></td><td></td></tr><tr><td>Phase duration, min</td><td>83 ± 59 n = 35</td><td>80 ± 58</td></tr><tr><td>FHR CTG signal quality %</td><td>94 ± 6</td><td>94 ± 3</td></tr><tr><td><strong>Expulsion</strong></td><td></td><td></td></tr><tr><td>Phase duration, min</td><td>16 ± 18 n = 22</td><td>11 ± 10</td></tr><tr><td>FHR CTG signal quality %</td><td>67 ± 20</td><td>62 ± 22</td></tr></tbody></table>

The results are expressed as mean ± standard deviation. ITT: intention to treat; FHR: fetal heart rate; MHR: maternal heart rate; CTG: cardiotocography.

## Performance

The overall PPA was 84.2 ± 10.4 % for the FHR and 80.7 ± 12.4 % for the MHR. For the FHR, the PPA was >80 % for all labor phases except the expulsion phase (44.7 %). The overall mean biases were 1.7 ± 2.4 for the FHR and 2.7 ± 8.1 for the MHR. For the FHR, the mean bias was lower than 2 bpm for all labor phases except expulsion (14.13 ± 15.04 bpm) (Table 3).

Figs. 2 and 3 represent two examples of FHR tracing comparisons between the TOCONAUTE and the CTG. In order not to induce bias in the example selection, we chose to present CTG recordings with lowest

<table><caption>**Table 3**
TOCONAUTE performance.</caption><thead><tr><th></th><th>PPA (%)</th><th>95 % CI (%)</th><th>Bias (bpm)</th><th>95 % CI (bpm)</th><th>ULoA (bpm)</th><th>LLoA (bpm)</th></tr></thead><tbody><tr><td><strong>FHR</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Overall (n = 35)</td><td>84.2 ± 10.4</td><td>80.7; 87.8</td><td>1.7 ± 2.4</td><td>0.9; 2.6</td><td>31.8 ± 11.9</td><td>-28.3 ± 10</td></tr><tr><td>Stage 1</td><td>85.5 ± 10.9</td><td>81.7; 89.2</td><td>1.7 ± 2.4</td><td>0.9; 2.6</td><td>30 ± 13.2</td><td>-26.6 ± 11.1</td></tr><tr><td>Latent (n = 33)</td><td>86.3 ± 11.5</td><td>82.2; 90.4</td><td>1.6 ± 2.7</td><td>0.7; 2.6</td><td>29.7 ± 15.8</td><td>-26.4 ± 13.1</td></tr><tr><td>Active (n = 26)</td><td>85.0 ± 12.1</td><td>80.1; 89.9</td><td>1.8 ± 3.9</td><td>0.3; 3.4</td><td>29.6 ± 14.8</td><td>-26 ± 13</td></tr><tr><td>Stage 2</td><td>76.0 ± 16.9</td><td>68.8; 83.1</td><td>2.6 ± 4.2</td><td>0.8; 4.3</td><td>50.3 ± 15.3</td><td>-33.7 ± 16.6</td></tr><tr><td>Descent (n = 23)</td><td>81.0 ± 15.7</td><td>74.2; 87.8</td><td>0.4 ± 6.8</td><td>-2.5; 3.4</td><td>30.8 ± 12.4</td><td>-30 ± 18.3</td></tr><tr><td>Expulsion (n = 23)</td><td>44.7 ± 23.5</td><td>34.6; 54.9</td><td>14.1 ± 15</td><td>7.6; 20.6</td><td>64.4 ± 22</td><td>-36 ± 24</td></tr><tr><td><strong>MHR</strong></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Overall (n = 35)</td><td>80.7 ± 12.4</td><td>76.5; 84.9</td><td>2.7 ± 8.1</td><td>-12.0; 5.5</td><td>41.1 ± 25.6</td><td>-36.5 ± 12.5</td></tr></tbody></table>

The results are expressed as mean ± standard deviation. FHR: fetal heart rate; MHR: maternal heart rate; PPA: positive percent of agreement; 95 % CI: 95 % confidence interval; ULoA: upper limit of agreement; LLoA: lower limit of agreement. bpm: beat-per-minute.
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Example (Patient 18, CTG overall quality = 97 %) of evolution of FHR (in bpm) against time (in minutes) computed by the TOCONAUTE (red) and CTG (blue) for the latent phase (upper left), active phase (lower left), descent phase (upper right), and expulsion phase (lower right).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Example (Patient 29, CTG overall quality = 97 %) of evolution of FHR (in bpm) against time (in minutes) computed by the TOCONAUTE (red) and CTG (blue) for the latent phase (upper left), active phase (lower left), descent phase (upper right), and expulsion phase (lower right).</figcaption></figure>

FHR quality (Patient 18) and highest FHR quality (Patient 29).

Discussion

To validate the TOCONAUTE technology for intrapartum fetal monitoring, we compared its performance to the CTG used in our clinical
practice. The TOCONAUTE provided FHR values with up to 80 % reliability, except for the expulsion phase. Concerning accuracy, the TOCONAUTE provided FHR values with an overall bias lower than 2 bpm, except for the expulsion phase. The low performance of the TOCONAUTE during the expulsion phase can be explained not only by the increasing rate of uterine contractions and maternal movements but also in part at the low performance of the gold standard CTG success rate of about 62 ± 22 % during this phase. Moreover, this phase has a very short duration compared to the other phases. The reliability and performance for the MHR were slightly lower than for the FHR. This result could probably be due to the increase of maternal movements during expulsion phase which could impact the signals quality. It could also be explained by the lower success rate of CTG for MHR computation (82 % versus 94 % for FHR).

In this study, 10 patients were excluded due to a bad TOCONAUTE signal quality. However, analysis of these 10 patients is of particular interest to understand the TOCONAUTE's limits of use. After obtaining several successive bad-quality signals (Patients 45, 46, 47), we noticed that the hard drive of the TOCONAUTE's PC was almost full, which could explain this poorer performance. For each patient, the TOCONAUTE files needed 1.11 ± 0.12 GB of storage space. This full hard disk problem could also explain the low performance obtained for Patients 43 and 44. After releasing memory space on the hard disk, the TOCONAUTE showed good reliability and accuracy. At the end of the study, the last six records had poor signal quality, possibly because the electrodes had dried. At the beginning of the study, the electrodes were delivered by the manufacturer in individual blisters, while the last 20 electrodes were provided in 10-electrode blisters. ECG electrodes are generally guaranteed for use 30 days after the blister opening and the last blister of electrodes had already been open for more than one month when the signal quality was poor (Patients 55–60). Future studies on the TOCONAUTE will therefore have to be performed with electrodes in individual packaging, and the disk space will need to be checked before each new patient is included.

Researchers have compared noninvasive fetal ECG (NI-fECG) to gold standard measures [18,20,21]. Although these studies used the same methodology to evaluate reliability and accuracy, they used fetal scalp electrodes (FSEs) as the gold standard and compared NI-fECG and DU performance. Cohen et al. demonstrated that NI-fECG was more reliable than DU, with PPAs of 81.7 ± 20.5 % and 73 ± 24.6 %, respectively [18]. They also found that NI-fECG was more accurate than DU, with 5.3 ± 2.4 bpm and 10.9 ± 5.8 bpm, respectively. The low reliability and accuracy of DU in their study can be explained in part by the fact that the participants (n = 75) had a high average BMI (32.6 ± 7.6 kg/m²). Euliano et al. used the same design as Cohen et al. and confirmed the superiority of NI-fECG to DU in terms of reliability (83.4 ± 15.4 and 62.4 ± 26.5, respectively) and accuracy (4.8 ± 2.0 and 14.3 ± 8.2, respectively) in a high-BMI population (average 35.1 ± 8.3) [20]. Lempersz et al. also compared NI-fECG and DU to FSEs in a more standardized population (BMI = 28.1 ± 5.9, 38/121 patients with BMI>30 kg/m²) [21]. They found that NI-fECG had an overall reliability of 86.8 ± 16.3 and an accuracy of − 1.46 ± 4.22 bpm, with limits of agreement of − 19.2; 26.3 bpm. In the same study, the success rate for DU was 82.8 ± 23.1 %. Lempersz et al. also demonstrated that NI-fECG reliability decreased from 87.8 ± 15.5 % to 68.7 ± 25.7 % between labor stages 1 and 2, whereas the accuracy remained equivalent.

Although we used DU as the gold standard instead of FSE, our results are in line with those of Lempersz et al. in terms of population, reliability, and accuracy. We also found a decrease in NI-fECG accuracy during stage 2 of labor, mainly due to signal loss during the expulsion phase. In our study, reliability and accuracy remained equivalent during the entire labor duration, except for the expulsion phase.

Our study has some limitations. First, the study was performed on a small population, and 25/60 patients were excluded from the study. However, most of the excluded patients (20/25) were excluded for

technical reasons that could be avoided in future research with technical improvements as well as modifications to the TOCONAUTE's instructions for use. Moreover, although our sample size was lower than those of Cohen et al., Euliano et al., and Lempersz et al. (35 versus 75, 71, and 103), the average recording duration was much longer in our study (519 min versus 255.6 min for Cohen et al., 223.6 min for Lempersz et al., and not provided for Euliano et al.) [18,20,21]. Second, the choice of DU as a gold standard could be criticized from a methodological point of view. However, the use of FSEs is not a standard of care in our center (only two patients required FSEs in this study), and the systematic use of such an invasive method in our patients could have posed ethical issues. Moreover, even though we used DU as the gold standard, our results are in line with those of studies using FSE.

## Conclusion

This study demonstrated the TOCONAUTE's reliability and accuracy for FHR estimation. The next step in this project is to validate the ability of fetal HRV to predict neonatal acidosis in a real clinical context. We will recruit a multicenter cohort of >750 patients during obstetric labor. This cohort will make it possible to compare HRV measurements computed from FHR records obtained from TOCONAUTE with arterial pH values at birth.

## Funding

This work was supported by the French National Research Agency (ANR) under grant number ANR-18-CE19-0015-01.

## Declaration of competing interest

Pierre Prot is employed by Bioserenity. The remaining authors have disclosed that they do not have any potential conflicts of interest.

## Acknowledgements

The authors would like to thank Mathilde Herbet from the CIC-IT, as well as all care providers at the Department of Obstetrics, for their kind help with this clinical trial.

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