Original Article
Monitoring Uterine Activity during Labor:
Clinician Interpretation of Electrohysterography
versus Intrauterine Pressure Catheter and
Tocodynamometry
Tammy Y. Euliano, MD1,2 Minh Tam Nguyen, MS3 Shalom Darmanjian, PhD3
John D. Busowski, MD, JD4 Neil Euliano, PhD3 Anthony R. Gregg, MD2
1 Department of Anesthesiology, University of Florida College of
Medicine, Gainesville, Florida
2 Department of Obstetrics and Gynecology, University of Florida
College of Medicine, Gainesville, Florida
3 OBMedical, Jonesville, Florida
4 Center for Maternal Fetal Medicine, Winnie Palmer Hospital for
Women and Babies, Orlando, Florida
Address for correspondence Tammy Y. Euliano, MD, Department of
Anesthesiology, University of Florida College of Medicine, 1600 SW
Archer Road, PO Box 100254, Gainesville, FL 32610
(e-mail: [email protected]fl.edu).
Am J Perinatol
Abstract
Keywords
► electrohysterography
► intrauterine pressure
► tocodynamometry
Objective The aim of this article was to compare clinical interpretation of uterine
activity tracings acquired by tocodynamometry and electrohysterography with the gold
standard, intrauterine pressure.
Study Design Using data from a previous study, subjects who had simultaneous
monitoring with all three uterine activity devices were included in this study. These were
parturients who required intrauterine pressure catheter (IUPC) placement for obstetric
indication. A Web-based application displayed scrolling 30-minute segments of uterine
activity. Two blinded obstetricians and two blinded obstetric nurses independently
reviewed the segments, marking uninterpretable segments and the peak of each
contraction. Interpretability was compared using positive percent agreement. False
positives are contractions marked in the noninvasive strip that have no corresponding
contraction in the IUPC strip. False negatives are the reverse.
Results A total of 135 segments, acquired during either Stage 1 (active labor) or Stage
2 (pushing), from 105 women, were included in this analysis. For all four observers, both
interpretability and sensitivity of electrohysterography exceeded that of tocodynamometry (p < 0.0001). This remained true for the obese population (96 segments).
Conclusion Compared with the IUPC, electrohysterography is more sensitive and
provides tracings that are more often interpretable than tocodynamometry for intrapartum monitoring; electrohysterography is also less affected by increasing maternal
body mass index.
Reliable uterine activity (UA) monitoring is essential for the
accurate interpretation of fetal heart rate tracings. Inadequate
contraction monitoring, particularly in the setting of oxytocininduced hyperstimulation, is common in litigation cases.1 The
high failure rate of external UA monitoring via tocodynamometry (Toco) in obese patients is well known.2,3 The traditional,
alternative placement of an intrauterine pressure catheter (IUPC)
carries small but real risks, particularly of infection.4
received
June 4, 2015
accepted after revision
December 16, 2015
Copyright © by Thieme Medical
Publishers, Inc., 333 Seventh Avenue,
New York, NY 10001, USA.
Tel: +1(212) 584-4662.
DOI http://dx.doi.org/
10.1055/s-0036-1572425.
ISSN 0735-1631.
Monitoring Uterine Activity during Labor
Euliano et al.
Recently, several groups have reported monitoring UA via
electrohysterography (EHG)5–8: recording of the electrical
activity of the uterus from the maternal abdominal wall.
Our group has similarly reported improved reliability of
EHG over Toco, using IUPC as the gold standard.5 The aim
of the current study is to evaluate the quality of each method’s
UA tracing by clinician assessment.
Methods
This study is an analysis of the UA data from a larger study
targeting fetal heart rate comparison of noninvasive fetal
electrocardiogram via abdominal electrodes, with fetal scalp
electrode (FSE) and ultrasound. The study was conducted at
UF Health (Gainesville, FL) and Winnie Palmer Hospital for
Women and Babies (Orlando, FL). The institutional review
boards at each institution approved the identical protocol
(project number 346-2010 on September 2, 2010) and all
subjects provided written, informed consent. Adult women
admitted to the labor and delivery suites at term (37 weeks
of gestation) in active labor with a singleton fetus in cephalic
presentation, without bleeding, uterine scar, or evidence of
chorioamnionitis, and with an IUPC and/or FSE in place for
obstetric indication, were eligible for inclusion. Maternal
positioning in the labor bed was not restricted. For the
present report, we include only those subjects with a valid
IUPC trace for at least 30 minutes during monitoring and who
met the following criteria: after allowing 10 minutes for
stabilization of all signals, at least one 30-minute UA strip
during Stage 1 with all three monitoring modalities and/or at
least 10 minutes during Stage 2.
Following skin preparation by gentle rubbing with abrasive gel, six 3-cm2 Ag/AgCl2 electrodes (Ambu; Glen Burnie,
MD) were placed on the maternal abdomen: four surrounding
the navel forming a diamond, each approximately 7.7 cm from
the navel. To reduce environmental noise, driven right leg and
common mode electrodes were placed on the subject’s left
flank. These electrodes should record little of the desired
signal and therefore were used to subtract noise common to
all electrodes. The electrodes were connected to the amplifier
in a monopolar fashion. Electrode positions were modified
slightly for each patient, as required by the location of the
tocodynamometer and ultrasound. Because the signals from
uterus and fetal heart are small, impedance (the resistance to
signal flow between electrode and skin) of each electrode was
measured (General Devices EIM-105 Prep-Check; Ridgefield,
NJ), and the skin was re-prepped to achieve an impedance
below 10 kΩ at each site.
The recorded signals were fed to a four-channel highresolution, low-noise unipolar amplifier. All four signals
were measured with respect to a reference electrode. The
amplifier design employed driven right leg circuitry to reduce
common mode noise between the patient and the amplifier.
The amplifier 3 dB bandwidth was 131 Hz.
Data from each patient included a UA channel from two
maternal–fetal monitors: Toco (Corometrics, GE Medical Systems, Waukesha, WI) and IUPC (Corometrics at UF Health, and
Avalon, Philips Healthcare, Andover, MA, at Winnie Palmer)
American Journal of Perinatology
sampled at 8 Hz with 8-bit resolution. These cardiotocographs reported the Toco- and IUPC-derived contraction
curves. Data also included output from four abdominal EHG
channels sampled at 500 Hz with 24-bit resolution.
To produce the EHG contraction curve, the four EHG
channels were band pass filtered between 0.2 and 1 Hz to
eliminate low- and high-frequency noise while preserving
the main contraction power, and adaptively combined based
on their signal-to-noise ratio to create the EHG UA trace. The
output was then down-sampled at 8 Hz and normalized to
scale the signal from 0 to 100 units for direct comparison with
Toco and IUPC tracings. All three UA curves were stored
electronically for subsequent analysis. The clinician caring
for the patient was blinded to all but the IUPC tracing. The
research assistant was instructed to adjust the Toco to acquire
the best possible tracing throughout the data collection.
Each collection for the first 10 minutes was discarded to
allow for stabilization of all signals. Each tracing was reviewed for the presence of Toco and IUPC waveforms, and
segments were discarded if either was absent. From the
remaining data, for Stage 1, 30-minute segments were identified by computer software that randomly selected the
starting point with the constraint that data segments not
overlap. One segment was selected for each strip with less
than 60 minutes of remaining duration, two for all others. For
Stage 2, all available 30-minute segments were selected for
analysis. If the Stage 2 segment duration was between 10 and
30 minutes, a continuous 30-minute segment, which includes
the length of Stage 2, was taken for analysis.
A Web-based application was developed to display the
segments of UA for evaluation. Four clinicians (two obstetricians and two labor and delivery nurses) participated. The
application presented a single UA segment at a time, in
random order. Clinicians were blinded to the source (IUPC,
Toco, or EHG) and subject. For each tracing, the clinician
marked contraction locations and uninterpretable UA regions. All markers were saved for future analysis. A 10-minute
example including all three modalities is provided in ►Fig. 1,
though during the study the segments were presented
individually.
Although we selected segments where the IUPC tracing
was clear, clinicians still identified uninterpretable sections.
Using the IUPC as the gold standard, where its tracing was
marked interpretable, the corresponding time periods of Toco
and EHG were investigated. Interpretability of the UA systems
was compared using positive percent agreement (PPA), defined as the percentage of time the noninvasive technology
(Toco or EHG) was also marked interpretable. Contractions
marked on the Toco/EHG signal that have a peak within
30 seconds of that marked on the IUPC signal are “consistent”
contractions.9 False positives are UA contractions marked by
clinicians in a Toco/EHG tracing and not in the corresponding
IUPC tracing. False negatives are UA contractions marked in
an IUPC tracing and not in the corresponding Toco/EHG
tracing. Contraction peak delay for each true positive was
computed by estimating the delay at which the cross-covariance between the contraction signal from Toco/EHG and IUPC
is maximized. For each stage of labor and each patient, the
Monitoring Uterine Activity during Labor
Euliano et al.
Fig. 1 Example of 10-minute segments of uterine activity from each modality marked by a single observer in the Web-based application.
Contraction peaks are marked with a vertical line. Uninterpretable segments are marked with a box.
performance characteristics were averaged across data segments when more than one was selected.
Demographic characteristics were compared using the
two-sample t-test. Descriptive statistics, reported as medians
with interquartile ranges (IQRs), were computed for all
performance characteristics for the entire dataset and for
those with body mass index 30. Either t-tests (normally
distributed) or Wilcoxon signed-rank tests (non-normally
distributed) were used to compare the quality of EHG and
Toco for PPA, false positives, false negatives, and contraction
delay (time between peak of IUPC and peak of EHG/Toco).
Additionally, median differences in contraction delay were
examined using the Hodges-Lehmann estimator. Statistical
tests were performed with Matlab R2013 (Mathworks, Natick,
MA) and JMP 11 (SAS Institute, Cary, NC) and were considered
statistically significant when p < 0.005, to account for multiple comparisons via a Bonferroni correction.
Results
Of the 167 subjects enrolled in the larger study, 105 met the
inclusion criteria for this report: 66 at UF Health and 39 at
Winnie Palmer. In total 13,129 minutes of UA was acquired
simultaneously with all three modalities. Stage 1 analysis
included 172 segments from 102 subjects. Stage 2 analysis
included 60 segments from 31 subjects.
Demographic characteristics of the subjects are listed
in ►Table 1. Subjects at UF Health were slightly younger
(p ¼ 0.017) than those at Winnie Palmer, and more Stage 2
data were acquired at UF Health, but otherwise the groups
were comparable.
Nearly all IUPC and EHG tracings were interpretable, with a
median of 0% for both IUPC and EHG of Stage 1 tracings
marked problematic and <4% of Stage 2 tracings (IUPC
¼ 3.5%; EHG ¼ 1.6%) for either device. Conversely, for more
than one-third of the time for both Stage 1 (46.5%) and Stage 2
(41.3%), the Toco tracing was marked uninterpretable; this
difference between EHG and Toco tracings was statistically
significant (p < 0.0001).
►Table 2 displays the interpretability (PPA) and contraction peak delay scores for EHG and Toco compared with the
standard. The PPA for EHG exceeded Toco for all tracings
(p < 0.0001), and for the obese subset (p < 0.0001). All
clinicians identified significantly more EHG contractions
Table 1 Demographic characteristics
Demographic variables
Total (n ¼ 105)
Mean (SD)
UF Health (n ¼ 66)
Mean (SD)
WP (n ¼ 39)
Mean (SD)
p
Age (y)
27.0 (5.7)
26.0 (5.7)
28.7 (5.4)
0.017
Gestational age (wk)
39.1 (1.2)
39.2 (1.3)
39.1 (1.1)
0.58
Body mass index
35.3 (8.9)
36.2 (9.5)
33.8 (7.6)
0.15
Stage 1 (min)
50.6 (14.0)
52.4 (13.2)
47.7 (14.9)
0.11
Stage 2 (min)
58.1 (44.5)
83.6 (55.4)
37.1 (13.1)
0.008
American Journal of Perinatology
Monitoring Uterine Activity during Labor
Euliano et al.
Table 2 Comparison between Toco and EHG for all subjects (na ¼ 135) and only in those with BMI 30
Variable
BMI 30
All subjects
Median
IQR
p
b
Median
IQR
<0.0001
PPA, %
EHG
100
2
Toco
54
60
<0.0001
100
2
55
61
<0.0001
Ctx delay(s)
pb
<0.0001
EHG
4.24
4.34
4.08
4.48
Toco
1.87
3.11
1.80
3.16
Abbreviations: BMI, body mass index; Ctx, contraction; EHG, electrohysterography; IQR, interquartile range; PPA, positive percent agreement; Toco,
tocodynamometry.
a
n ¼ number of distinct Stage I patients þ number of distinct Stage II patients.
b
p-Value from Wilcoxon signed-rank test.
consistent with IUPC than they did in the Toco tracings
(►Table 3). While they also recorded more false-positive
contractions with EHG, overall mean sensitivity was higher
with EHG (90%) as compared with Toco (46%). EHG contraction delay (median ¼ 4.24, IQR ¼ 4.34), relative to IUPC, was
slightly larger than for Toco (median ¼ 1.87, IQR ¼ 3.11),
with a Hodges-Lehmann median difference ¼ 2.00 (95% confidence interval [CI]: 1.53–2.48).
To assess interobserver differences for each outcome, separate Wilcoxon signed-rank tests were conducted for each pair
of observers for Toco and EHG. No statistically significant
differences were observed for false positives, false negatives,
or contraction delay for Toco or EHG. For PPA, there was
evidence for interobserver differences. Notably, the second
obstetric nurse had significantly (p < 0.001) higher PPA ratings
(median [IQR] ¼ 81% [85]) compared with the other observers
(median [IQR] for obstetrician 1 ¼ 45% [91]; median [IQR] for
obstetrician 2 ¼ 55% [81]; median [IQR] for obstetric nurse
1 ¼ 55% [88]) for Toco. However, in secondary analyses that
removed the second obstetric nurse’s ratings, PPA was still
significantly higher (p < 0.0001) in EHG (median [IQR]
¼ 100% [0]) as compared with Toco (median [IQR] ¼ 51%
[88]). Comparisons between Toco and EHG for each observer
are presented in Appendix A (►Tables A1–A5).
Discussion
In this comparative study, experienced clinicians found EHGderived UA tracings more often interpretable than Toco
tracings. The sensitivity of EHG was also superior, with
approximately 90% of all IUPC-detected contractions similarly
marked on the EHG tracing, nearly double that of Toco. In
using frequency of contractions to titrate oxytocin,10 reliable
detection provides a clinical advantage. Of note, reviewers of
EHG tracings identified slightly more contractions than were
seen on the corresponding IUPC tracings. Whether this was
maternal or fetal movement generating a confounding electrical signal or weak contractions not detected by the IUPC is
unclear. Work continues on the algorithm to reduce these
false positives.
The correlation between IUPC and Toco tracings, including
contraction frequency, amplitude, and duration, has been
investigated. Miles et al11 reported “good correlation” of
contraction frequency detected by the Toco (r ¼ 0.75) in 20
patients with median BMI of 31.8. This was, however, simply a
count of contractions over a 2-hour time period, without
assessment of whether individual contractions corresponded
to those detected by IUPC. Meanwhile they found poor
correlation between Toco and IUPC in regard to contraction
Table 3 Cumulative contractions, false positives, and false negatives for each device
Cumulative
Contractions (mean SD)
b
Consistent contractions (mean SD)
False positives (mean SD)
b
False positives (%)
False negatives (mean SD)
False negatives (%)
a
b
b
IUP
Toco
EHG
pa
2,493.3 27.0
1,226.5 35.9
2,448.2 78.4
<0.0001
1,182.3 23.2
2,242.5 27.0
<0.0001
44.3 23.8
205.8 61.1
0.003
2%
8%
<0.0001
1,311.5 34.5
251.3 39.7
<0.0001
53%
10%
<0.0001
p-Value differences between tocodynamometry and electrohysterography, either by t-test for continuous variables or z-test for proportions.
Averaged across all four clinicians.
b
American Journal of Perinatology
Monitoring Uterine Activity during Labor
amplitude and duration.11 Several groups, including our own,
have reported the superiority of EHG over Toco,5,7,9 especially
in the obese.12 Although contraction amplitude does not
directly correlate between EHG and IUPC,6 a relationship
does exist13–16 that could be used to generate comparable
tracings.
Clinically, labor monitoring largely remains a visual interpretation of segments that pair UA and fetal heart rate.
Several classification schemas exist, but numerous studies
report poor inter- and even intraobserver interpretation
reliability.17,18 Besides monitoring the labor itself, the importance of UA tracing extends to the relative timing of decelerations, yet few have reported on visual interpretation of UA
tracings. Bakker et al19 studied Toco and IUPC tracings during
the last 2 hours of Stage 1 and throughout Stage 2. Two of the
authors classified tracings into adequate, “a recognizable and
reliable pattern during the complete registration,” and inadequate. The latter was subdivided into absent UA pattern and
“recognizable but unreliable pattern,” if there was poor
calibration. They report that 40% of IUPC tracings were
adequate throughout the studied period compared with
<2% of Toco. In the remainder of tracings, inadequate registration was due almost exclusively to failure of calibration for
the IUPC, whereas for Toco, the majority was an absent UA
tracing. The authors conclude the superiority of IUPC “if
external monitoring does not provide an adequate UA trace.”
Hadar et al9 found a similar rate of interpretable tracings
for EHG (87%) and IUPC (95%), while Toco was significantly
lower (68%). Identified contraction frequency was also lower
for Toco than for EHG relative to IUPC.
In a report by Reinhard et al,7 four gynecologists reviewed EHG and Toco tracings from first and second stages
of labor. Tracings were graded as “adequate” or “inadequate,” with the former defined as “a recognizable and
reliable pattern with a baseline calibration at or below
20 mm Hg (20%).” Only contractions that occurred during
“adequate” periods were counted. The group found no
inadequate EHG tracings compared with inadequate Toco
during approximately 10% of the simultaneous recording
time. This in part explains the higher number of detected
contractions for EHG. Perhaps more interesting is the
variability in contraction counts for the Toco. The authors
attribute this not to a difference in the duration of segments
labeled adequate by each observer but rather to the greater
difficulty interpreting Toco tracings.
Performance of the Toco is affected by the low sensitivity of
the device, particularly in the obese; the necessity of calibration; and the frequent need for position adjustment to overlie
the fundus. A limitation of this study is selection bias. All
subjects had an IUPC placed for obstetric indication, often
because of an inadequate tracing. The protocol did require
continued attempts to acquire an acceptable Toco tracing, but
it was not used for clinical decision making. This does not
diminish the finding that EHG functioned well in the setting
of an inadequate Toco. In settings such as preterm labor and
premature rupture of membranes, where an IUPC is contraindicated, or undesirable, EHG can provide a much-needed
alternative to Toco.
Euliano et al.
Weaknesses of this study include the use of only four
clinicians, one of whom reported a significantly higher PPA
for EHG than the others. Because removal of her data did not
change the results, and inclusion only increased the validity of
EHG, the authors believe it unlikely that inclusion of additional reviewers would substantially alter the conclusions.
The lack of a standard definition for “uninterpretable” may be
a weakness. Reviewers were instructed that all subjects were
in active labor and having contractions, and to mark uninterpretable regions. Further they were instructed to “assess the
traces as if this were a real life situation.” Three of the four
clinicians had similar PPA and therefore likely shared a
definition of “interpretable.” A more strict definition may
have lessened the variability, but the goal was to mimic real
clinical use.
Since these data were collected, the LaborView (OBMedical, Newberry, FL) device has acquired FDA approval. Currently, pricing information is unavailable. The disposable
electrode array will be more expensive than a reusable
tocodynamometer, and likely more expensive than an IUPC.
Off-setting this cost would be a potential reduction in the
need to place IUPCs, together with a reduction in complications from that placement. An intangible cost saving would be
reduction in nursing time adjusting the Toco position or
calibration. The device is wireless, facilitating maternal movement such as position changes in bed, and movement to a
chair. Recording during ambulation introduces substantial
noise to the signal and is not currently a feature of the device.
To our knowledge, this is the first report of expert visual
comparison of EHG, Toco, and IUPC tracings. Our team of
clinicians found the EHG to be superior to Toco, even in obese
patients, approaching the quality of IUPC tracings for reporting qualitative UA.
Conflict of Interest
N. E. is Chief Technical Officer and M. T. N. and S. D. are
employees of OBMedical, Jonesville, FL. T. Y. E. is married to
N. E. and is listed on patents filed for some of the
technology described in this article. The other authors
report no conflict of interest.
Note
The study was supported by OBMedical and the University
of Florida.
Acknowledgments
The authors would like to thank the clinicians who reviewed the segments: Kathryn Davidson, MD; Daniel
Kushner, MD; Joann Tanner; and Susan Nickel; the authors
would also like to thank the data collector at UF Health;
Teresa Lyles, PhD; and Michele L. Real at Winnie Palmer
Hospital for Women and Babies; our statistician, Terrie
Vasilopoulos; and our editor, Corey Astrom. Finally, we
would also like to thank the nursing and physician staff at
both hospitals.
American Journal of Perinatology
Monitoring Uterine Activity during Labor
Euliano et al.
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Monitoring Uterine Activity during Labor
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Appendix A
Table A1 Interpretability comparison: positive percent agreement for all subjects (na ¼ 135)
Subject and device
Median
IQR
1.000
0.450
0
0.907
<0.0001
Obstetrician 1
EHG
Toco
<0.0001
Obstetrician 2
EHG
Toco
1.000
0.505
0
0.857
<0.0001
Obstetric Nurse 1
EHG
Toco
1.000
0.550
0.050
0.880
1.000
0.805
0
0.848
<0.0001
Obstetric Nurse 2
EHG
Toco
p
Abbreviations: EHG, electrohysterography; IQR, interquartile range; Toco, tocodynamometry.
a
n ¼ number of distinct Stage I patients þ number of distinct Stage II patients.
Table A2 Interpretability comparison: positive percent agreement for subjects with BMI 30 (na ¼ 96)
Subject and device
Median
IQR
1.000
0.435
0
0.890
EHG
Toco
1.000
0.515
0
0.918
<0.0001
Obstetrician 2
EHG
Toco
<0.0001
Obstetric Nurse 1
EHG
Toco
1.000
0.560
0.058
0.875
<0.0001
Obstetric Nurse 2
EHG
Toco
p
<0.0001
Obstetrician 1
1.000
0.875
0
0.838
Abbreviations: BMI, body mass index; EHG, electrohysterography; IQR, interquartile range; Toco, tocodynamometry.
a
n ¼ number of distinct Stage I patients þ number of distinct Stage II patients.
Table A3 Sensitivity for all subjects (na ¼ 135)
Subject and device
Median
IQR
1.000
0.400
0.140
0.850
<0.0001
Obstetrician 1
EHG
Toco
<0.0001
Obstetrician 2
EHG
Toco
1.000
0.450
0.123
0.873
<0.0001
Obstetric Nurse 1
EHG
Toco
1.000
0.460
0.170
0.800
1.000
0.450
0.120
0.770
<0.0001
Obstetric Nurse 2
EHG
Toco
p
Abbreviations: EHG, electrohysterography; IQR, interquartile range; Toco, tocodynamometry.
a
n ¼ number of distinct Stage I patients þ number of distinct Stage II patients.
American Journal of Perinatology
Monitoring Uterine Activity during Labor
Euliano et al.
Table A4 Sensitivity for subjects with body mass index 30 (na ¼ 96)
Subject and device
Median
IQR
1.000
0.400
0.150
0.850
p
<.0001
Obstetrician 1
EHG
Toco
<.0001
Obstetrician 2
EHG
Toco
1.000
0.500
0.113
0.883
<.0001
Obstetric Nurse 1
EHG
Toco
0.920
0.500
0.170
0.788
<.0001
Obstetric Nurse 2
EHG
Toco
1.000
0.460
0.110
0.780
Abbreviations: EHG, electrohysterography; IQR, interquartile range; Toco, tocodynamometry.
a
n ¼ number of distinct Stage I patients þ number of distinct Stage II patients
Table A5 Absolute contraction delay in seconds for all subjects
Subject and device
na
Median
IQR
Hodges-Lehmann
median difference
Hodges-Lehmann
confidence intervalb
Obstetrician 1
EHG
Toco
97
4.20
1.40
4.30
2.43
2.40
2.00–2.90
Obstetrician 2
EHG
Toco
104
4.15
1.80
4.13
3.50
2.00
1.50–2.50
Obstetric Nurse 1
EHG
Toco
108
4.25
1.85
4.18
3.28
2.00
1.50–2.50
Obstetric Nurse 2
EHG
Toco
110
4.40
1.70
4.50
4.10
2.1
0
1.60–2.60
Abbreviations: EHG, electrohysterography; IQR, interquartile range; Toco, tocodynamometry.
a
n ¼ number of distinct Stage I patients þ number of distinct Stage II patients who presented uterine activity contractions.
b
95% two-sided confidence interval.
American Journal of Perinatology