† These authors contributed equally.
Background: Myometrial contractile activity can be evaluated by
recording uterine electromyography (EMG) non-invasively from the abdominal
surface. Uterine EMG has been shown to detect contractions during labor as
reliably as tocography (TOCO) and intrauterine pressure catheters. To evaluate
whether changes in uterine EMG throughout the first stage of labor correlate with
advancing cervical dilatation. Methods: Uterine EMG was recorded from the
abdominal surface for 30
minutes in 32 women during the first stage of labor at term. Women were divided
in three groups according to cervical dilatation at the time of EMG recording:
Myometrial contractile activity can be evaluated by recording uterine electromyography (EMG) non-invasively from the abdominal surface [1, 2, 3]. Uterine EMG has been shown to detect contractions during labor as reliably as tocography (TOCO) and intrauterine pressure catheters [4, 5, 6]. In addition, EMG yields important information on changes in electrical properties of the myometrium characteristic of labor [7, 8]. Several studies demonstrated that uterine EMG predicts onset of labor at term and preterm more accurately than other methods in clinical use today [9, 10, 11, 12, 13]. Uterine EMG properties have also been shown to differentiate active vs. latent labor in women presenting with regular contraction at term [7, 14].
No uterine studies to date evaluated uterine EMG as a means of assessing progress of labor in the first stage, i.e., from onset of regular painful contractions to full dilatation of uterine cervix. This assessment currently still mostly depends on vaginal examinations to estimate cervical change and TOCO to assure adequate frequency of contractions [15, 16]. Both methods have several drawbacks. TOCO became the standard of care more than 40 years ago without ever undergoing vigorous clinical trials [17]. It only measures change in shape of the abdominal wall as a function of uterine contractions and, as a result, is a qualitative rather than a quantitative method [17]. Monitoring uterine activity with TOCO alone has been proven not to be helpful in assessing progress of labor [17, 18]. On the other hand, estimating cervical dilatation by digital vaginal exam is very subjective and measurements vary significantly among caregivers [19, 20]. Moreover, vaginal exams may cause discomfort and pain, and can also be a source of infection [21, 22]. Therefore, a more objective and less invasive method to assess labor progress in the first stage of labor would be very helpful to clinicians and could increase satisfaction of laboring women with intrapartum care.
The objective of the study was to evaluate whether changes in uterine EMG throughout the first stage of labor correlate with advancing cervical dilatation.
Women at 37 0/7 to 41 6/7 weeks of gestation with singleton pregnancies and fetuses in cephalic presentations admitted with regular uterine contractions to labor ward of the Department of Perinatology, University Medical Center Ljubljana, Slovenia, were included in the study. All women included provided written informed consent for study participation. The National Medical Ethics Committee approved the study (reference number: 137/02/10).
Uterine EMG measurements were performed in labor ward. Four electrodes were arranged symmetrically around the navel. Uterine EMG was recorded for 30 min using the SureCall Monitor (Reproductive Research Technologies, Houston, Texas, USA). Recordings were analyzed with Chart 5 software (ADInstruments, Castle Hill, Australia). In order to exclude most components of motion, respiration, and cardiac signals, and to more clearly discern “bursts” of uterine electrical activity, EMG signals were digitally filtered (band-pass 0.3 to 1.00 Hz). Data were sampled at 100 Hz. Fourier transform was used to obtain the power density spectrum (PDS) of each burst. The PDS curve is a function of frequency and represents the relative contribution of each frequency to the signal. Peak PDS frequencies of bursts analyzed in recording were averaged to obtain a mean peak PDS frequency for each patient.
Different clinicians blinded to the results of uterine EMG measurements
performed cervical examinations. Women were classified into three groups
according to cervical dilatation at the time of EMG recording:
Thirty-two women were included in the study: 4 (13%) were dilated
Characteristic | Cervical dilatation |
Cervical dilatation 3–5 cm | Cervical dilatation 6–10 cm | p |
(N = 4) | (N = 19) | (N = 9) | ||
BMI (kg/m |
28 |
31 |
27 |
0.07 |
Nulliparity | 4 (100%) | 9 (47%) | 5 (56%) | 0.12 |
Maternal age (years) | 32 |
31 |
29 |
0.71 |
Gestational age (weeks) | 39 |
39 |
39 |
0.86 |
Labor augmentation with oxytocin | 1 (25%) | 9 (47%) | 3 (33%) | 0.62 |
Means with standard deviations or Number N (%) are shown; BMI, body mass index. |
Uterine EMG PDS peak frequencies differed significantly in the three groups
(p
Comparison of uterine electromyography (EMG) power density
spectrum (PDS) peak frequency in three groups of women in the first stage of
labor according to cervical dilatation. PDS peak frequency differed
significantly among groups (p
The main finding of the study is that uterine EMG PDS peak frequencies increase with increasing cervical dilatation. This indicates potential effectiveness of uterine EMG as a means for assessment of labor during the first stage.
Continuing trend towards higher EMG activity as labor progresses is in line with
earlier findings with intrauterine pressure measurements [23]. However, increase
in uterine EMG PDS peak frequency with increasing cervical dilatation does not
merely reflect stronger contractions late in the first stage, but suggests
changes in electrical properties of the myometrium (e.g., increase in cell
coupling through gap junctions, increased frequency of electrical signals etc.)
throughout the first stage of labor. These changes can be detected non-invasively
by recording uterine EMG from the abdominal surface. Our results are in
accordance with previously published studies on accuracy of uterine EMG in
predicting labor at term and preterm [9, 12, 13]. Maner et al. [9, 24, 25] showed high sensitivity and specificity of EMG for predicting onset of labor
at term. Similarly, Lucovnik et al. [12] found high diagnostic values of
uterine EMG for identifying women with preterm contractions at
Relatively high rates of labor augmentation with oxytocin should be taken into
account when interpreting our results. Active management of labor with high-dose
oxytocin regimen (initial oxytocin infusion of 2 to 5mU/min with increments every
20–30 min until a maximum dose of 40 mU/min is reached) is common obstetric
practice at our institution [26, 27]. Although there are no direct data on
effects of oxytocin on uterine EMG PDS peak frequencies, it stands to reason then
these could be affected by oxytocin administration. Pajntar et al.
[28, 29, 30] showed an increase in uterine cervical EMG activity after oxytocin
administration and animal studies have found changes in uterine and oviduct EMG
associated with oxytocin use. Moreover, Maul et al. [23] have
described a correlation between uterine EMG properties and intrauterine pressure.
Our conclusions on increase in EMG PDS peak frequencies with advancing cervical
dilatation were probably not influenced by oxytocin use, as there were no
significant differences augmentation rates between the three study groups.
However, further studies are needed to determine whether same findings apply to
spontaneous (non-augmented) labors alone. Parity is another characteristic that
merits discussion. Although there were no statistically significant differences
in parity among groups, it has to be noted that all women in the
The main limitation of our study is the relatively small number of women included. The cross-sectional nature of the study can also be viewed as a limitation. We did not track EMG changes in individual women longitudinally. Instead, we compared three groups of women with different degrees of cervical dilatation at EMG recording. Studies of longer EMG recordings throughout labor will be needed to confirm or refute our results. Nevertheless, our results may be viewed as a proof of concept that uterine EMG could potentially be used as a noninvasive tool for assessing labor progress that could help minimizing the number of unnecessary and potentially harmful vaginal examinations.
ML and ATB designed the research study. ATB performed the research. KG provided help and advice on the writing an article. NSP, ER and ML analyzed the data and write an article. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.
The National Medical Ethics Committee approved the study (reference number: 137/02/10). Patient consent was waived as it is a retrospective study without risks to the participants, evaluating and improving current clinical management.
We would like to express our gratitude to all pregnant women who agreed to participate in the study. Thank you to all the peer reviewers for their opinions and suggestions.
This study received no external funding.
The authors declare no conflict of interest.