Eleven women were studied at the time of elective caesarean section at 38–40 weeks gestation. Written informed consent was obtained from each participant. The study was approved by the South Sheffield Research Ethics Committee. Participants were excluded if they had any of the following: previous cervical surgery, multiple pregnancy, ruptured fetal membranes prior to Caesarean section, reproductive birth defects, or cervical dilatation > 3 cm.
Cervical EIS studies
The impedance measuring device consists of tetrapolar probes of different sizes (3 mm and 12 mm, Figure 1) that attach to a single channel electrical impedance measurement system (the Sheffield Tissue Impedance Meter Mk 4.0, University of Sheffield, Figure 2).
Cervical resistivity was measured with the 3 and 12 mm probes by two observers conversant with the technique. Prior to these experiments an electronic balance was used to pre-calibrate the two forces of application to be employed in a modification of the method described by Gonzalez-Correa . These forces equated to approximately 0.7 Newton ("soft"), and 2.2 Newton ("firm"). The latter approximates to the degree of force routinely employed by us in our clinical experiments. Prior to each experiment, the two observers undertook a balance calibration to ensure that the force applied to the measuring probe was consistent between observers and equated to the approximate force of application being studied. In preparation for Caesarean section, each participant was anaesthetised by means of spinal block, positioned in lithotomy, and a Cusco's speculum used to gently expose the cervix. Using a cotton swab, any thick vaginal mucus obscuring the view obtained was gently removed avoiding contact with the epithelial surface.
For each probe, measurements were taken by each of the two operators from the 12 o'clock position on the anterior lip of the cervix. Each observer took 2 measurements 1–2 minutes apart applying the probe to the cervix with each of the two pre-determined degrees of force. The order of the probes was randomised to avoid systematic bias. The impedance measuring device is connected wirelessly to a computer with a Matlab® software interface (The Mathworks Inc., Natick, MA, USA) for data capture and display. The basic design of this measurement system in vivo has been described previously . Transfer impedance values measured in ohms (Ω) were simultaneously obtained at 14 electrical frequencies ranging from 76 Hz to 625 kHz increasing in octave steps and stored in ASCII format.
The data were initially tested for normality of distribution by means of the Kologorov-Smirnov test. As this suggested that the data were not normally distributed, a logarithmic transformation was applied to all the data before statistical analysis. For each, repeatability (intra-observer variability) and reproducibility (inter-observer variability) measures were derived for each of the 14 frequencies studied at the two forces of application: the intra-class correlation coefficient (ICC), the coefficient of variation (CV), the repeatability and reproducibility standard deviations and 95% limits of agreement of repeat measurements, all of which methods have been extensively employed in the comparison of repeated measurements between observers [13–15]. The ICC is a method of measuring inter-rater reliability and is computed from a one-way analysis of variance (ANOVA) on the log transformed data as described by Shrout and Fleiss . The effect of the following variables on the repeatability and reproducibility of EIS measurements were investigated: observer, probe size and application force. Intra-class correlation coefficients were determined using one-way random single measure for intra-rater analysis and two-way mixed model, absolute agreement definition for inter-rater analysis. Reliability was regarded as excellent if ICC > 0.75, fair to good if 0.4 ≤ ICC ≥ 0.75, and poor if ICC ≤ 0.4 . The coefficient of variation measures the dispersion of a distribution and is useful in comparing data sets with very different means as occurs in EIS measurements between individuals. The CV was derived as the ratio of the standard deviation of each group of measurements to the mean . The repeatability and reproducibility standard deviations were derived from Bland-Altman plots  which also summarised the limits of agreement between: a) 2 measurements taken by each observer at a defined transducer pressure and b) 2 measurements taken by two observers at a defined transducer pressure . When comparing data derived for soft pressure to that for firm pressure for each probe, a two-tailed paired Student t test was also performed.
Statistical analysis was performed using SPSS for Windows (version 15.0, SPSS Inc, Chicago IL) and the MedCalc (version 9 Mariakerke, Belgium) statistical packages.