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VOL. 43 | NO. 3 | SEPTEMBER - DECEMBER 2022 | PP 6-14
RESEARCH ARTICLE
E-LOCATION ID: 1290
dx.doi.org/10.17488/RMIB.43.3.1
ib
Vaginal Electrical Impedance Detects the Fertile Window in Healthy
Women: a Pilot Study
La Impedancia Eléctrica Vaginal Detecta la Ventana Fértil en Mujeres Sanas: un Estudio Piloto
José Alfonso Cruz-Ramos1 , Joe Klepich2 A, Martha Eloisa Ramos-Márquez1 A, Gerardo García-Gil3 A,
Luis Ilich Vladimir Guerrero-Linares3 A, Gabriela del Carmen López-Armas3 A B
1Universidad de Guadalajara, Centro Universitario de Ciencias de la Salud - México
2School of Informatics, master's in Information and Data Science, University of California, Berkeley - United States
3Centro de Enseñanza Técnica Industrial, Guadalajara, Jalisco - México
ABSTRACT
This work analyzes the electrical impedance (EI) measurement of cervical mucus (CM) using a device to determine
the fertile window. In this prospective and longitudinal study, fourteen healthy women aged 18 to 44 were enrolled
to evaluate three menstrual cycles. EI was measured through a medical device inserted into the vagina for two min-
utes daily. Patients were monitored by urine luteinizing hormone (LH) strip, blood collection, and vaginal ultrasound
to visualize the dominant follicle. Finally, the predictive EI capacity was validated by the receiver operating charac-
teristic (ROC) of anovulatory vs. ovulatory impedances. The peak of LH was 35.7 (±4.5) mUI/ml and the dominant
follicle size was 15.45 mm (±0.559). There were statistical dierences in EI measurements between the follicular
and luteal phases vs. the ovulation phase (p<0.0361 and p<0.0160). After data normalization, an area under the
ROC curve (AUC) of 0.713 (P value= 0.0253), a Youden J index of 0.4545Ω, a sensitivity of 63.6%, and a specicity of
81.8% were found. Low EI in the ovulatory period belongs to the LH ovulatory peak and follicular release. EI can be
used for ovulation monitoring, birth control, or promoting pregnancy as a safe and innocuous method.
KEYWORDS: Electrical impedance, cervical mucus, ovulation, luteinizing hormone
REVISTA MEXICANA DE INGENIERÍA BIOMÉDICA | VOL. 43 | NO. 3 | SEPTEMBER - DECEMBER 2022
7
RESUMEN
Este trabajo analiza la medición de la impedancia eléctrica (IE) del moco cervical (MC) mediante un dispositivo para
determinar la ventana fértil. En este estudio prospectivo y longitudinal, se incluyeron 14 mujeres sanas de 18 a 44
años para evaluar tres ciclos menstruales. La IE se midió a través de un dispositivo médico colocado en la vagina du-
rante dos minutos diarios. Las pacientes fueron monitoreadas con una tira de hormona luteinizante (LH) en orina, re-
colección de sangre y ultrasonido vaginal para visualizar el folículo dominante. Finalmente, la capacidad predictiva
de IE fue validada por la curva ROC (receiver operating characteristic) de impedancias anovulatorias vs. ovulatorias.
El pico de LH fue de 35.7(±4.5) mUI/ml; el folículo de tamaño dominante fue de 15.45 mm (±0.559). Se encontraron
diferencias estadísticas para la medición de la IE de las fases folicular y lútea versus la fase de ovulación (p<0.0361
y p<0.0160). Después de la normalización de los datos, se encontró un área bajo la curva ROC (AUC) de 0.713 (valor
de P = 0.0253), un índice de Youden J de 0.4545 Ω, sensibilidad del 63.6 % y especicidad del 81.8 %. La IE baja en el
período ovulatorio que pertenece al pico ovulatorio de LH y liberación folicular. La IE se puede utilizar para el control
de la ovulación, el control de la natalidad o la promoción del embarazo como método seguro e inocuo.
PALABRAS CLAVE: Impedancia eléctrica, moco cervical, ovulación, hormona luteinizante
Corresponding author
TO: Gabriela del Carmen López-Armas
INSTITUTION: Centro de Enseñanza Técnica Industrial
ADDRESS: Calle Nueva Escocia #1885, Col. Providencia
5ta. Sección, C. P. 44638, Guadalajara, Jalisco, México
CORREO ELECTRÓNICO: glopez@ceti.mx
Received:
20 June 2022
Accepted:
18 August 2022
José Alfonso Cruz-Ramos et al. Vaginal Electrical Impe dance Detects the Fe rtile Window in Healthy Wom en: a Pilot Study 8
INTRODUCTION
Cervical mucus CM is produced by secretory cells in
endocervical glands and discharged into the endocervi-
cal conduct. CM is composed mostly of water up to 90%;
the rest consists of mucins that confer viscoelastic gel
properties, amino acids, ions, cholesterol, lipids, glucose,
polysaccharides, soluble proteins, and enzymes 1 2 3. In
normal conditions, the secretion of CM changes quantita-
tively and qualitatively through the hormonal cycle stage.
The secretion of CM depends on the hormonal cycle:
women produce 20-60 mg of CM per day until they
reach approximately 600 mg in the ovulation period, a
crucial change for the transport of spermatozoa to the
mature follicle 4. This pattern occurs in fertile women,
but in infertile females, the panorama is quite different
5. Several hormones participate in ovulation: gonado-
tropin-releasing hormone GnRH acting from the
hypothalamus; follicle-stimulating hormone FSH and
luteinizing hormone LH secreted by the anterior pitu-
itary gland, and estradiol and progesterone produced in
the ovary. In the initial phase of the menstrual cycle,
the estrogens stimulate FSH production and release; by
positive feedback, FSH increases during the irst half of
the cycle. In the middle of the cycle, the rise in FSH and
LH triggers ovulation 6. Later, the surge in FSH and LH
generates negative feedback in gonadotropin secretion,
and the hormone levels fall during the second half of
the cycle. If pregnancy is absent, the secretion of estro-
gens and progesterone starts to diminish after ovula-
tion, and menstrual bleeding occurs. Hormonal fluctu-
ations throughout the menstrual cycle are the main
factor influencing ovulation 7 .
Ovulation is currently detected through blood tests and
intravaginal ultrasound to identify the dominant ovar-
ian follicle 8. EI has been widely used in biological appli-
cations since 1925, with signiicant advances during the
past one hundred years 9 1 0 1 1; actually, it may be a
more reliable and convenient detection tool. Although
EI has been used to assess cervical neoplasia or cervical
ripening in pregnant and non-pregnant women 12 13 14 ,
it has barely been applied to detect ovulation 15 16 17 18
19 20. Hence, this work aims to determine the ovulation
window via measuring the impedance of CM by a mini-
mum invasive EI device. Our principal objective was to
assess the feasibility of correlating impedance values
with serum concentrations of estradiol, FSH, LH, pro-
gesterone, prolactin, and total testosterone, as well as
intravaginal ultrasound, to observe and measure ovar-
ian follicle release. All laboratory tests were performed
according to each patient menstrual cycle to precisely
detect the ovulation period or fertile window.
MATERIALS AND METHODS
This pilot, prospective, longitudinal protocol aimed to
identify variations of EI in cervical mucus at different
phases of the reproductive cycle. Fourteen healthy
women aged from 18 to 44 years and with a complete
gynecological-obstetric history were included. Each
participant provided written consent for blood proce-
dure, intravaginal ultrasound, and device use. Internal
Committee for Research and Follow-up of Research
Projects CIESPI of the Technical Industrial Teaching
Center approved the study with the number PI-07-18-
20. All selected participants had a normal hormonal
proile, did not use hormonal contraceptives, DIU dis-
positive, or any vaginal treatment such as lubricants,
creams, or ointments for at least two months, and were
willing to take protective measures during sexual inter-
course. All women had regular pap smear reports.
During the study, two participants were excluded by a
hormonal disturbance in the second cycle and another
one for pregnancy, staying only eleven. With prior train-
ing for device and mobile app use, all women auto-mea-
sured impedance with the Kegg device for two minutes
daily in the morning. Measurements were made for
three consecutive menstrual cycles starting the irst
day after bleeding in each cycle. The hormonal activity
was monitored in urine, blood samples, and ovulation
via vaginal ultrasound Figure 1- c. For the correct use
of the device, the dorsal position was obligatory.
REVISTA MEXICANA DE INGENIERÍA BIOMÉDICA | VOL. 43 | NO. 3 | SEPTEMBER - DECEMBER 2022
9
Electric impedance measurement
We used kegg device, model K-1 Lady Technologies,
Inc., San Francisco, CA, powered by a 50mAh battery
that includes an electronic impedance circuit made
with gold-plated stainless-steel electrodes, a main-
board to transfer all information via Bluetooth to
mobile application Android and IOS and a vibration
motor for patient feedback. The device is covered with
medical-grade silicone for safety and comfortability
and has been thoroughly tested according to IEC/ISO
standards Figure 1 a-b. For proper usage, the device
is inserted into the vagina to reach the cervical fluid
pool.
Urine and blood sample hormonal
measurements
On the same day the patients started using the EI
device, they tested their LH concentration with the
Clearblue® strip S.P.D., Swiss Precision Diagnostics
GmbH, Geneve, Switzerland on the irst-morning
urine daily. Once the strip test was positive, blood was
collected by a venous puncture for hormonal determi-
nation of LH, FSH, estradiol, prolactin, and total tes-
tosterone employing immunoquimioluminiscence
Ortho, Vitros 5600, Inc. California, USA. Progesterone
was determined in another blood sample by the same
methodology on day 21 of each cycle.
Vaginal ultrasound
The following day after the positive LH strip, the
woman underwent a vaginal ultrasound SonoAceR3,
Samsung Health Care. Seul, South Korea to verify the
amount and size of follicles in both ovaries.
Characteristic of EI device
The impedance measurement of the kegg device is
based on a balanced bridge whose voltage is read by
the microprocessor ADC. The processor generates a 1
FIGU RE 1. (a) KeggTM device for EI measurement of CM. (b) The mobile application data register. (c) Timeline. Arrow 1.
All subjects must finish vaginal bleeding to start measuring impedance. Arrow 2. All participants began to use the LH strip
until getting a positive result and continued using the device. Arrow 3. Serum quantification of hormones in the blood once
positive urine strip: LH, FSH, Estrogen, total testosterone, and prolactin. Vaginal ultrasound performed within 24 to 48
hours after LH peak. Arrow 4. Serum quantification of progesterone. This scheme was repeated for three menstrual cycles.
José Alfonso Cruz-Ramos et al. Vaginal Electrical Impe dance Detects the Fe rtile Window in Healthy Wom en: a Pilot Study 10
kHz signal at a known voltage which is used to excites
one electrode called the tip, the voltage is then read
on the other electrode called the ring through an
op-amp. This procedure is then reversed, whereby the
second electrode is excited and read by the irst. The
processor performs a calculation based on the known
values of the other legs of the bridge, and a reference
value excitation voltage is then established to deter-
mine the resistance. The measurement is made multi-
ple times to create a sample of set of 50 readings which
is then averaged for a inal impedance value.
The communication process is activated when the
device receives a command from the mobile app via
Bluetooth; ifty readings are required during a single
measurement session. These readings are transferred
to an Android mobile app via Bluetooth Low Energy;
then, the data are saved on the cloud and processed to
determine which readings fall within the iftieth per-
centile and calculate a median value labeled as the
inal impedance value. These values are entered into
the database for each user's session. The Bluetooth is
turned off during the actual measurement process.
Statistical Analysis
Data analysis was carried out using GraphPad Prism LLC
version 8.01 software San Diego, CA and Medcalc soft-
ware Ltd Ostend, Belgium. Results are presented as
means or medians. The raw impedance values of each
phase were compared between them and to those of the
other phases in the same woman and other subjects' val-
ues via the Mann-Whitney U test with Holm-Sidak cor-
rection and an alpha error of 0.05. The level of signii-
cance was set at P <0.05. Data were normalized as follows:
The means of the impedance values of the follicular
Equation 1 FOL and luteal Equation 2 LUT phases of
each cycle were obtained with the following formula:
1
!"# =$+ %+ &…
#'( =$+ %+ &…
!"#− = !"#
#'( − = #'(
)*+,- = $)*+,- − )*+,-
This means the impedance value of the day of ovula-
tion Impov LH peak of the corresponding cycle was
subtracted from Equations 3 and 4. The ovulation
impedance values of 3 cycles were added, and the mean
was obtained, which was subtracted from each individ-
ual ovulatory impedance measurement. Equation 5.
With these values, multiple comparisons were made
using the Friedman test and the ROC curve to compare
the ovulatory phase against non-ovulatory phases
luteal and follicular.
RESULTS AND DISCUSSION
No participants reported any injury or collateral effect
using the device to measure impedance in cervical dis-
charge or discomfort by venipuncture for blood tests.
Subjects' metrics
Table 1 describes the mean age and standard devia-
tion SD of the eleven participants at the beginning of
the study, as well as blood values of LH, FSH, estradiol,
prolactin, and total testosterone taken on ovulation
day; only progesterone content was measured on day
21st of each menstrual cycle. Finally, to verify the
ovulation, we also perform ultrasonography 24 to 48
hours after LH serum peak to visualize follicle size and
number Figure 2.
CM raw impedance values
In total, there were 676 impedance measurements
from cervical discharge from eleven participants. In
Figure 3, we divided the menstrual cycle into three
2
!"# =$+ %+ &…
#'( =$+ %+ &…
!"#− = !"#
#'( − = #'(
)*+,- = $)*+,- − )*+,-
3
!"# =$+ %+ &…
#'( =$+ %+ &…
!"#− = !"#
#'( − = #'(
)*+,- = $)*+,- − )*+,-
4
!"# =$+ %+ &…
#'( =$+ %+ &…
!"#− = !"#
#'( − = #'(
)*+,- = $)*+,- − )*+,-
5
!"# =$+ %+ &…
#'( =$+ %+ &…
!"#− = !"#
#'( − = #'(
)*+,- = $)*+,- − )*+,-
REVISTA MEXICANA DE INGENIERÍA BIOMÉDICA | VOL. 43 | NO. 3 | SEPTEMBER - DECEMBER 2022
11
phases: follicular, ovulatory LH peak, and luteal. Our
results show that the difference between follicular and
ovulatory phases were statistically signiicant in imped-
ance measurements median 349.1Ω, and median
311.8Ω, P value= <0.0361 concerning the luteal phase
vs. ovulatory phase median 350.3Ω, and median
311.8Ω, P value= <0.016. Finally, we performed Holm-
FIGU RE 4. Box and whisker plot. Impedance normalized
data of follicular, ovulation, and luteal phases. (*) Statistical
differences were found for the impedance measurement
between all phases (P < 0.0435)—Friedman test.
Sidak correction with an alpha of 0.05 for follicular and
luteal phase comparations against the ovulation phase
adjusted P value= <0.03 and adjusted P value= <0.02.
CM normalized impedance values
After data normalization, the Friedman test yielded a
value of P = <0.0435 Figure 4.
FIGU RE 3. Box and whisker plot. Impedance raw
measure of follicular, ovulation, and luteal phases. (*)
Statistical differences were found in the impedance
measurement of the follicular phase. U de Mann test.
FIGU RE 2. Vaginal ultrasound of one patient.
Blue marks signal the dominant follicle of 1.95 cm.
The yellow and purple marks represent the ovarian
width and length in centimeters.
TAB LE 1 . Characteristics of the study subjects.
Table 1
Parameters
Values
Total participants (n)
11
Age (years)
30.6(±8.2)
LH (mUI/ml)
35.7 (±4.5)
Estradiol (pg/ml)
179.3 (±14.2)
Prolactin (ng/ml)
22.60 (±1.93)
Total Testosterone (ng/ml)
0.4109 (±0.025)
José Alfonso Cruz-Ramos et al. Vaginal Electrical Impe dance Detects the Fe rtile Window in Healthy Wom en: a Pilot Study 12
FIGURE 5. ROC curves from normalized data of
impedance measurements of the non-ovulation and
ovulation (LH peak day) periods (p value= <0.0253).
To evaluate the device as a valuable method to distin-
guish the ovulation phase from follicular and luteal
phases of the menstrual cycle, we obtained a receiver
operating characteristic ROC curve under bootstrap
conidence interval 1000 iterations; random number
seed: 978, an area under the ROC curve AUC of 0.713
value of p = <0.0253, and a Youden index J of 0.4545
with a sensitivity of 63.64% and speciicity of 81.82%
Figure 5. With these impedance measurements, we
can discriminate between no ovulation period follicular
and luteal phases and ovulation period LH peak day.
This study describes the application of electrical
impedance measurement to investigate conductivity
changes in CM during the menstrual cycle in healthy
women of reproductive age. Our results revealed a lower
impedance during the ovulatory phase compared with
the follicular and luteal phases. It is well known that pH
changes in CM play a pivotal role in sperm survival since
during the ovulation period, the mucus becomes less
acidic with a nearly neutral pH, and sperm is thus pro-
tected, leading to changes in electrical impedance 2 1.
In this sense, some research has monitored the day-
to-day changes in CM by EI devices. Previous reports
measured salivary or vaginal electrical resistance SER
or VER with a CUE Fertility Monitor. In general, results
show that the peak in LH occurred 4 to 5 days after the
peak measurement in SER, and this had a strong posi-
tive correlation r = 0.94 with the urinary LH surge. As
for VER values, these diminished along with the pre-
ovulatory phase from day -3 until day 0. In this report,
the VER mean at day 0 correlated with the LH surge;
consequently, the nadir was the lowest value on the
CUE Fertility Monitor. Moreno, et al. 13. concluded that
CUE Fertility Monitor has potential for use in natural
family planning NFP. In contrast, a prospective study
on the reliability of the CUE Fertility Monitor 22 that
analyzed sixteen cycles with signals i.e., SER and
VER for the beginning and the end of the fertile
period, could not ind signals in two cycles; therefore,
the researchers did not recommend the CUE Fertility
Monitor utilizing the current algorithm for NFP.
In this work, the EI device was used only once a day
to measure CM. The main objective was to simplify the
daily CM evaluation making it practical and comfort-
able for women. The results of this study indicate a low
impedance value coincident with the LH peak and the
appearance of the dominant follicle, as seen by ultra-
sonography. However, several factors mentioned
below in order of importance can affect the imped-
ances recorded by the device and should be considered
by the end-user. First, end-users must follow the man-
ufacturer's instructions for the Kegg device for two
complete ovulatory cycles.
The methodology carried out in this study provides
enough data to the APP for analysis. Second, it is nec-
essary to constantly monitor the LH in urine or serum
to determine a correlation between the impedance
values obtained in the irst three months when the LH
test is positive. The observation of the LH peak and the
impedance value is fundamental because each woman
has her own hormonal rhythms that influence the
composition of the mucus in the different phases,
there are ultrastructural changes such as the increase
REVISTA MEXICANA DE INGENIERÍA BIOMÉDICA | VOL. 43 | NO. 3 | SEPTEMBER - DECEMBER 2022
13
in pore size of the network of ibers that make up the
mucus during the ovulatory phase that allows a greater
flow of substances producing variations in resistivity
and conductance 23. Another factor is the water con-
tent of the mucus, which reaches up to 99% during
ovulation, which has an impact on the decrease in
impedance in this phase, as opposed to the follicular
and luteal phases where the water content is 90 to
95%. Finally, mucus content and density may also
have an influence, as there are noticeable changes in
the follicular phase, with cervical mucus being more
abundant and dense in the luteal phase compared to
the ovulatory phase 2 4. This last property depends on
hydration, electrolyte content, exercise routine, nutri-
tional habits, age, some vaginal infections, stressful
conditions, etc. So, each woman needs to learn to
know her hormonal levels and observe in time how
they can affect CM production 25. Third, in this study,
users were asked to use protection in all sexual encoun-
ters to avoid direct contact of the device with the
semen, as this could modify the impedance value.
Once the proper cervical mucus values are gained, the
recommendation is to allow 10-12 hours between the
sexual encounter and the impedance measurement.
CONCLUSIONS
This study shows that the kegg device, model K-1,
helps to identify the LH ovulatory peak. Indeed, serum
and urinary increases of LH, together with the ultraso-
nographic documentation of a dominant follicle,
detect a woman's fertile window with a sensitivity of
63.6% and speciicity of 81.8%. Measurement of CM
electric impedance with the Kegg device is more reli-
able than daily monitoring of urinary LH test alone,
making this method cost-effective along with the
ease-of-use beneits. Further studies exploring differ-
ent disorders polycystic ovary syndrome, endometri-
osis, hyperprolactinemia, premature ovarian failure,
or imbalance in thyroid hormone that can affect the
sensitivity and speciicity of EI ovulation values are
required.
ETHICAL STATEMENT
This study followed the ethical principles regarding
human experimentation proposed by the Helsinki dec-
laration; all women were informed about the proce-
dures, risks, and beneits to obtain the written consent
of voluntary participation. The protocol was approved
by the Internal Committee for Research and Follow-up
of Research Projects CIESPI of the Tech nical Industrial
Teaching Center with the number PI-07-18-20.
CONFLICT OF INTEREST
Lady Technologies, Inc supported this project.
ACKNOWLEDGMENT
Despite the health contingency, we are deeply grate-
ful to each of the women who participated in this study
and the health personnel of the different institutions.
Also, we want to thank MD. Ma. de la Paz Jacinto-
Orozco our OB/GYN of the protocol, Engr. Mario
Antonio Mercado-Cisneros for all protocols communi-
cation between KEGG device and APP and Dr. Horacio
Rivera-Ramírez for his review of the manuscript.
AUTHOR CONTRIBUTIONS
J.A.C.R. performed formal analysis, carried out mathemat-
ical and statistical methods for the analysis, participated in
the writi ng, reviewing, and editing t he different states of the
manuscript. J.K. oversaw the development of the project,
obtained funding and resources, designed the clinical pro-
tocol, carried out formal analysis, and developed mathe-
matical, statistical and computational analyses. M.E.M.R.
participated in the writing reviewing and editing of the
manuscript at different stages, pre and post publication.
G.G.G in charge of data curation. L.I.V.G.L. designed the
methodology, carried out experiments collected data and
evidence. G.C.L.A. conceptualization of the project’s objec-
tives and goals, designed the methodology, carried out sta-
tistical analyses oversaw the general project, participated in
the writing of the initial manuscript, carried out and partic-
ipated in the translation to English language. All authors
reviewed and approved the inal version of the manuscript.
REVISTA MEXICANA DE INGENIERÍA BIOMÉDICA | VOL. 43 | NO. 3 | SEPTEMBER - DECEMBER 2022
14
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