10

General Section

Labor induction using modified metreurynters plus oxytocin
at an institution in Japan: a retrospective study
N. Matsumoto1,2, M. Osada1, C. Matsumoto1, Y. Gomi1,3, S. Era1,3, H. Udagawa1,
N. Suzuki1, S. Takahashi1
1Department of Obstetrics and Gynecology, Fukaya Red Cross Hospital, Saitama
2Department of Obstetrics and Gynecology, Tatedebari Sato Hospital, Gunma
3Department of Obstetrics and Gynecology, Saitama Medical Center, Saitama Medical University, Saitama (Japan)

Summary
Objective: The authors evaluated the effectiveness and safety of “neo-metoro” or “mini-metoro” metreurynters plus oxytocin for labor
induction and assessed differences in parturition outcomes, according to the metreurynter used at induction initiation. Materials andMethods: The authors retrospectively reviewed 146 consecutive women with live singleton pregnancies, and who underwent induction.
Parturition outcomes were vaginal delivery achieved within the planned schedule (VDPS), vaginal delivery finally achieved (VDF), andinduction-to-delivery interval (IDI). Women were divided into neo-metoro, mini-metoro, and without metreurynter groups based on metreurynter use at induction initiation. The authors examined the relationships of metreurynter groups with factors, parturition outcomes,
and adverse events. In 113 women who underwent two-day induction, the authors calculated IDI and adjusted odds ratio (AOR) for
achieving delivery per unit time. Results: VDPS rates were 65% in nulliparous and 81% in multiparous women. VDF rates were 78% innulliparous and 96% in multiparous women. AORs for VDPS were 0.30 in nulliparous women and 0.18 in Bishop score (BS) 1–3 class.
AORs for VDF were 0.04 in BS1–3 class and 0.14 in BS4–5 class. In 113 women undergoing two-day induction, AORs for achieving
delivery per unit time were 0.45 in nulliparous women, 0.46 in obese women, and 0.48 in BS1–3 class. Neo-metoro use at induction initiation tended to reduce IDI. Conclusions: Labor induction using these metreurynters plus oxytocin is safe and effective. The advantagesof neo-metoro over mini-metoro use at induction initiation remain unclear; neo-metoro use at induction initiation may reduce IDI.
Key words: Bishop score; Cervical ripening; Cesarean delivery; Foley balloon catheter; Premature rupture of membranes.

Introduction
Labor induction in Japan
Labor induction (LI) is one of the most commonly
practiced obstetric intervention worldwide. LI comprisesof cervical ripening and uterine augmentation. Transvaginal dinoprostone (prostaglandin E2) or misoprostol(prostaglandin E1) is often used to mature the unfavorable cervix for inducing uterine contractions [1]. Sometimes insufficient contractions necessitate uterine
augmentation, which is often achieved by intravenous
drip administration of oxytocin.
However, transvaginal prostaglandins are not licensed for
LI in Japan where mechanical methods are mainly used tomature the unfavorable cervix. Transcervical Foley balloon
catheters are used for a mechanical method worldwide. Furthermore, their use often necessitates subsequent oxytocin
administration because these catheters induce active laborless effectively than prostaglandins [2]. In Japan, several
types of metreurynters are commercially available andwidely used for balloon catheterization on LI.

Revised manuscript accepted for publication January 24, 2013
Clin. Exp. Obst. & Gyn. - ISSN: 0390-6663
XLI, n. 1, 2014
doi: 10.12891/ceog16022014

Recently, the Japanese Society of Obstetrics and Gynecology (JSOG) issued a guideline for LI [3, 4]. Nevertheless, clinical studies on LI in Japan, especially those on LI
with metreurynter use, are scarce [5, 6]. Each institution
has its own LI protocols based on experience and basic clinical policy. At the present institution, Fukaya Red Cross
Hospital, Saitama, Japan, the authors use the discoid “neometoro” and spherical “mini-metoro” metreurynters. Bothare transcervical balloon catheters, designed specificallyfor LI, made of silicon gum. Neo-metoro forms an approximately six to seven by four-cm disc when inflated with 80100 ml sterilized water, while mini-metoro forms anapproximately four-cm sphere when inflated with 40 ml ofsterilized water. Both devices have a moderately hard shaft,making them easier to insert into the cervix than a Foley
catheter. The shaft of mini-metoro is slightly thinner thanthat of neo-metoro. Both metreurynters are inserted throughthe cervix and placed between the internal cervical os andamniotic membrane or fetal head. Because of the dimensions of these types of metreurynters, neo-metoro is difficult to use when cervical dilatation is less than one cm or
more than four cm and mini-metoro is difficult to use whencervical dilatation is less than 0.5 cm or more than three
cm. The authors use osmotic dilators if insertion of any me-

11

N. Matsumoto, M. Osada, C. Matsumoto, Y. Gomi, S. Era, H. Udagawa, N. Suzuki, S. Takahashi

Table 1. — Relationship of the metreurynter groups with characteristic factors, parturition outcomes, and adverse events in the 146
women.
Factors, parturition outcomes, adverse events

Age (years)
ParityMaternal height (cm)Maternal weight (kg)Maternal BMI (kg/m2)
Gestational age (weeks)
at induction †Indication forlabor induction

Nulliparous
Multiparous

Post-term
PROM
PRH
Complications
Prevention ofa precipitate labor
OthersMetreurynter groups
Total
n = 146

Without metreurynters
n = 12 (8%)

Neo-metoro
n = 99 (68%)

31.4 ± 4.7
93
64%
53
36%
159.0 ± 5.3
65.7 ± 10.1
25.9 ± 3.8
40.1 (35.0-41.4)

33.2 ± 4.9
33%67%
156.8 ± 5.6
65.2 ± 10.7
26.3 ± 3.6
39.2 (35.0-41.0)

31.4 ± 4.6
30.7 ± 4.7
0.29
61%
83%
0.005*
39%17%
159 ± 5.3
159.6 ± 5.0
0.30
66.2 ± 10.3
64.3 ± 9.7
0.65
26.1 ± 3.9
25.3 ± 3.7
0.50
40.1 (36.0-42.4） 40.9 (36.1-42.1) 0.016*

58
45
17
10

40%
31%
12%
7%

0%
42%
8%
17%

39%
31%
14%
7%

54%
26%
6%
3%

4
12

3%
8%

25%
8%

1%
7%

0%
11%

42%
58%
7.8 ± 1.6
–

51%
49%
5.3 ± 1.5
21%

71%
29%
4.4 ± 1.6
20%

0.065
0.065
<0.001*0.88

100%
6.7 ± 3.673%
6.7 ± 3.077%
7.5 ± 2.4

0.61‡
0.44

0%
100%
8.7 ± 11.4
67%
17%
17%
100%
0%
0%
67%

91%
9%
21.3 ± 12.7
77%
11%
12%
42%
33%
25%
77%

100%
0%
27.5 ± 13.8
63%
14%
23%
75%
25%
0%
54%

0.50

83%
3,294 ± 603
8%
8%
8%

88%
3,091 ± 428
8%
1%
3%

77%
3,017 ± 376
6%
0%
6%

Maternal complications(including PRH or GDM) No
80
55%
Yes
66
45%
Bishop score at induction initiation
5.3 ± 1.7
Achieving vaginal delivery
28
19%
without oxytocin
Oxytocin administration Yes
111
76%
Maximum dose
6.9 ± 3.0
(mu/min)Planned scheduleTwo-day induction 125
86%
One-day induction 21
14%
Induction-to-delivery interval (h)21.7 ± 13.6
Mode of delivery
Normal106
73%
Vacuum or forceps 18
12%
Cesarean
22
15%
Indication for cesarean
Arrest of labor
13
9%
delivery
NRFS
64%
Others3
2%
Vaginal delivery achieved
103
71%
during the planned scheduleVaginal delivery finally achieved
124
85%
Neonatal birth weight (g)3,090 ± 434
Low Apgar score (≤ 7 at 1 or 5 min)
11
8%
NICU admission
2
1%
Fever during induction of labor (≥ 38.0°C)
64%Mini-metoron = 35 (24%)

p< 0.001*

0.016*‡
0.53
0.32
0.041*
0.31
0.17
0.90
0.089
0.59

Data are presented as number, mean ± standard deviation, or percentage (in each metreurynter grouping) unless otherwise indicated.
BMI: body mass index; GDM: gestational diabetes mellitus; NICU: neonatal intensive care unit; NRFS: non-reassuring fetal status; PRH: pregnancy-related
hypertension; PROM: premature rupture of membranes.
*Statistically significant. †Median (range). ‡Comparison of the neo-metoro group with the mini-metoro group (because all cases in the without metreurynter group wereadministered oxytocin and induced by the one-day schedule).

treurynter into the closed and firm cervix proves impossible, but this did not occur during the study period. Active
labor is often not obtained after metreurynter use, and insuch cases, the authors administer oxytocin via an intravenous drip for uterine augmentation.
Here the authors evaluated the effectiveness and safetyof methods using neo-metoro or mini-metoro in combination with oxytocin for LI. They also assessed the differences in parturition outcomes according to metreurynter
type used at induction initiation.

Materials and Methods
Indications and informed consent
Major indications for LI are premature rupture of membranes(PROM) and post-term pregnancy. Other indications include
pregnancy-related hypertension, complications, prevention of
precipitate labor, oligohydramnios, fetal abnormality, and societal indications. Anesthesia for labor and delivery (planned oroccasional) is not performed at the present institution. In addition, the authors do not handle breech vaginal delivery, twin
vaginal delivery, or vaginal delivery after prior cesareans. LI isindicated when spontaneous labor onset does not occur within

12

Labor induction using modified metreurynters plus oxytocin at an institution in Japan: a retrospective study

Table 2. — Adjusted odds ratios calculated by multivariate logistic regression analysis assessing the relationship between eachfactor and VDPS and VDF parturition outcomes.
Factors

Parturition outcomes
VDPS
AOR

Age
ParityMaternal BMI (kg/m2)
Indication for labor induction
Complications
Neonatal birth weight (g)Bishop score

Metreurynter groups

Five-year elevating
Nulliparous
< 22
≥ 29
Post-term
PROM
OthersYes≥ 3,500
1–3
4–5
6–7
8–10
Mini-metoro
Neo-metoro
Without metreurynters

0.64
0.30
1.2
0.87
1
1.8
0.73
0.74
0.53
0.18
0.66
1
0.79
1
2.2
0.94

95% CIp

0.07–5.2
0.10–0.85
0.34–4.9
0.23–3.5
0.65–5.5
0.22–2.4
0.28–2.0
0.16–1.7
0.04–0.65
0.21–1.9

0.68
0.024*
0.76
0.85
1
0.25
0.60
0.54
0.26
0.008*
0.45

0.14–5.1

0.80

0.85–5.8
0.11–7.7

0.11
0.96

VDF
AOR

1.3
N/A†
0.87
0.76
1.00
1.1
0.45
0.33
0.04
0.14
1
N/A‡
1
1.5
0.21

95% CIp

0.69–2.6

0.40

0.11–7.4
0.11–5.7

0.89
0.78

0.26–3.9
1.0
0.19–7.7
0.89
0.12–1.6
0.22
0.06–1.6
0.17
0.002–0.33 0.002*
0.007–0.89 0.036*

0.40–5.8
0.004–9.6

0.53
0.40

AOR: adjusted odds ratio; BMI: body mass index; CI: confidence interval; N/A: not available; PROM: premature rupture of membranes; VDF: vaginal delivery finallyachieved; VDPS: vaginal delivery achieved during the planned schedule.
*Statistically significant. †The VDF rate in the multiparous women was too high to analyze the relation to parity, so the factor of parity was extracted in the multivariate
analysis. ‡The VDF rate in the BS8–10 class (which was 100%) was too high to analyze.

Table 3. — Multivariate Cox proportional hazards model
assessing the effect of each factor on induction-to-delivery
interval in 113 women who underwent LI via Schedule A of thetwo-day induction.
Factors

Age (years)
ParityBMI (kg/m2)

AOR

Five-year elevating 0.82
Nulliparous
0.45
< 22
0.64
≥ 29
0.46
Indication for
Post-term1labor induction
PROM
1.4
Others0.94
Complications
Yes
0.69
Neonatal birth weight ≤ 3,500 g
0.57
Bishop score
1–3
0.48
4–5
0.76
6–7
1
8–10
0.56
Metreurynter inserted Mini-metoro
1
at induction initiation Neo-metoro
1.5

95% CIp

0.63–1.07 0.14
0.25–0.80 0.006*
0.31–1.3 0.20
0.21–0.92 0.027*
0.81–2.4 0.22
0.47–1.8 0.85
0.39–1.2 0.18
0.30–1.03 0.063
0.22–0.96 0.038*
0.46–1.3 0.28
0.12–1.7

0.33

0.89–2.6

0.13

Calculated hazard ratios were regarded as odds ratios for achieving delivery perunit of time, with higher hazard ratio indicating earlier delivery.
LI: labor inducton; AOR: adjusted odds ratio; BMI: body mass index; CI:
confidence intervals; PROM: premature rupture of membranes.
*Statistically significant.

24 hours of PROM. For safety, cesarean is prioritized over LI in
cases with severe complications or in cases wherein vaginal
delivery seems to be hard to achieve. Induction initiation occurs
after careful consideration of maternal condition and provisionof informed consent.
Standard schedules
The basic procedure for LI is metreurynter insertion in combination with subsequent oxytocin administration. Standard
schedules of LI comprise two-day and one-day inductions.

Schedule selection depends on cervical maturity at induction initiation. In the two-day induction, in principle, metreurynters areinserted in the afternoon (three to five pm) of the day before thetarget delivery day (Schedule A); sometimes in the morning of
the day before the target delivery day (Schedule B). Moreover,
the two-day induction is divided into mini-metoro insertion andneo-metoro insertion at induction initiation. The one-day induction is selected when a woman has a comparatively favorable
cervix. Moreover, a neo-metoro is inserted prior to oxytocin if
possible (Schedule C); no metreurynter is inserted when thecervix is sufficiently favorable (Schedule D). Inserted
metreurynters are removed when extruded through the cervix
into the vagina.
If contractions are insufficient in the morning of the target
delivery day, oxytocin is administered to obtain active labor. The
initial oxytocin dose is two milliunits (mu)/min (12 ml/h of fiveunits oxytocin in 500 ml solution), followed by 3.3 mu/min (20ml/h of oxytocin solution) 30 min later, with a dose incrementof 1.7 mu/min (10 ml/h of oxytocin solution) at least every 30
min; the maximum dose administered is 20 mu/min (120 ml/h
oxytocin solution).
Manual cervical dilatation, manual membrane stripping, andartificial amniotomy are administered with discretion. Artificial
amniotomy will only be performed when delivery (vaginal orcesarean) will likely be achieved within several hours. If delivery is not foreseen to be achieved by five to seven pm based onlabor progress during the afternoon of the target delivery day,oxytocin administration is stopped. At this time, without indications for cesarean delivery and if mother and fetus are considered fit to tolerate further attempts, the authors advise rest before
attempting LI the following day.
To monitor fetal heart rate and uterine contractions, theauthors use cardiotocography with external transducers at least
20 min before and after metreurynter insertion, intermittently
during early labor, and continuously during active labor or oxytocin administration. Recently, the authors continuously monitor
fetal heart rate in cases with neo-metoro insertion, in accordance

N. Matsumoto, M. Osada, C. Matsumoto, Y. Gomi, S. Era, H. Udagawa, N. Suzuki, S. Takahashi

13

Figure 1. — Relationship between BS and average IDI, VDPS, VDF, and characteristic factors in the 146 women.

with JSOG recommendations when using a transcervical balloon
catheter inflated to 41 ml or more [4].
In addition, antibiotics are administered when metreurynters
are inserted, PROM occurs, Group B streptococcus has been
detected, and prevention of infection is indicated. Broad-spectrum penicillins such as piperacillin sodium are often administered intravenously at the present institution.
Methods
The authors retrospectively reviewed 146 consecutive womenwith live singleton pregnancies, cephalic presentation, no prior cesarean sections, and who underwent LI at the present institution.
They obtained characteristic factors, parturition outcomes, and adverse events from the obstetric database and the medical records.
To estimate LI efficacy, the authors defined parturition outcomes as vaginal delivery achieved within the planned schedule(VDPS), vaginal delivery finally achieved (VDF), and induction-to-delivery interval (IDI). VDPS included women who
achieved vaginal delivery, including vacuum-assisted or forceps
deliveries, by six pm of the target delivery day. VDF included
women who eventually achieved vaginal delivery, including
vacuum-assisted or forceps deliveries, however excluding
cesarean section. Cesarean delivery was considered as failed
induction. IDI was defined as the interval in hours from induction initiation to delivery, including cesarean delivery. Induction
initiation was defined as the time of metreurynter insertion or
oxytocin administration in women where metreurynters were notused. Women were divided into the following three groups
according to metreurynter use at induction initiation: the neometoro group, the mini-metoro group, and the “without
metreurynters” (WOM) group.
Bishop score (BS), measured upon metreurynter insertion or
oxytocin initiation, was used to judge cervical maturity at induction initiation. Women were divided into BS1–3, BS4–5, BS6–
7, and BS8–10 classes for analysis and interpretation of results.
The following three adverse events were assessed: low Apgar
score (≤ 7 at one or five min), admission to a neonatal intensive
care unit, and maternal fever during induction (≥ 38°C).
The relationship of metreurynter groups with characteristic

factors, parturition outcomes, and adverse events were assessedusing the Chi-square (χ2), Student’s t-test, and Mann–Whitney U
test. To assess the effect of each factor on VDPS and VDF, theauthors used multivariate logistic regression analysis.
IDI was assessed in 113 women who underwent LI via Schedule A. The authors constructed Kaplan–Meier survival curves
and analyzed using univariate log-rank tests separately for nulliparous and multiparous women. Moreover, they used the multivariate Cox proportional hazards model to assess the effect ofeach factor on IDI. Calculated hazard ratios were regarded as
odds ratios for achieving delivery per unit time, with higher
hazard ratio indicating earlier delivery.
Statistical analyses were performed using JMP 8 and all tests
were two-tailed. A p < 0.05 was considered statistically significant.

Results
LI was conducted by the two-day induction in 86%(125/146) women and by the one-day induction in 14%(21/146). Furthermore, 77% (113/146) underwent LI via
Schedule A [56% (82/146) with neo-metoro, 21% (31/146)
with mini-metoro], 8% (12/146) via Schedule B, 6% (9/146)
via Schedule C, and 8% (12/146) via Schedule D. Therefore, some metreurynter was used for LI in 92% (134/146).
Table 1 shows the relationship of metreurynter groups
with characteristic factors, parturition outcomes, and adverse events in the 146 women. With regards to characteristic factors, nulliparity, late gestational age, and post-term
pregnancy were higher and PROM and BS were lower inthe mini-metoro group than other two groups. Completely
opposite characteristics were evident in the WOM group,
while intermediate characteristics were observed in the neometoro group. With regards to parturition outcomes, IDI in
the mini-metoro group was longer than that in the neo-

14

Labor induction using modified metreurynters plus oxytocin at an institution in Japan: a retrospective study

Figure 2. — Kaplan–Meier survival curve and log-rank test assessment of IDI in 113 women who underwent LI via Schedule A of
the two-day induction. Markers (triangles/circles) indicate cesarean delivery, considered censored cases in the analyses. (A) multiparous women (n = 39); (B) nulliparous women (n = 74).

metoro group. The VDPS rate was highest in the neometoro group, followed by that in the WOM and minimetoro groups. The VDF rate in the mini-metoro group was
lower than that in the other two groups, but the difference
was not significant. Adverse events were observed in eachgroup but the differences were not significant. Severe incidents, such as severe neonatal asphyxia, neonatal death,uterine rupture, or umbilical cord prolapse, were not observed in this study.
Figure 1 illustrates the relationship of BS classification
with parturition outcomes and characteristic factors in the
146 women. As BS increased, average IDI decreased (p =0.011) and the VDF rate increased (p = 0.003). Metreurynter use was lowest in women belonging to the BS8–10 class(p < 0.001), and the most of the women in the mini-metorogroup belonged to the BS1–3 class (p = 0.037).
There is an obvious relationship between parturition outcome and parity. The overall VDPS rate was 71%(103/146) and was higher in multiparous than in nulliparous women [81% (43/53) vs 65% (60/93); p = 0.035].
Furthermore, the overall VDF rate was 85% (124/146) andwas higher in multiparous than in nulliparous women [96%(51/53) vs 78% (73/93); p = 0.004].
Table 2 shows the results of multivariate logistic regression analysis used to assess the relationship between factors, VDPS, and VDF. The VDF rate in multiparous women
was too high to analyze its relationship with parity, andhence, parity was excluded from multivariate analysis. The
significant adjusted odds ratio (AOR) for VDPS was 0.30
in nulliparous women and 0.18 in women belonging to theBS1–3 class. AOR for VDPS in the neo-metoro group was
2.2, indicating an insignificant positive tendency. The significant AOR for VDF was 0.04 in women belonging to theBS1–3 class and 0.14 in women belonging to the BS4–5
class. With regards to VDF, no advantage of neo-metoro
use was evident.
IDI was assessed in 113 women who underwent LI via
Schedule A. Kaplan–Meier curves for multiparous women(n = 39, Figure 2A) were identical between the neo-metoro
and mini-metoro groups. In contrast, Kaplan–Meier curves
for nulliparous women (n = 74, Figure 2B) showed that the
proportion of women who did not achieve delivery within
24 hours of induction initiation differed between the neometoro and mini-metoro groups (68% vs 49%), but this difference demonstrated not to be significant by log-rank test.
To assess the effect of each factor on IDI, AOR for achieving delivery per unit time was calculated by Cox proportional hazards model (Table 3). Significant AOR was 0.45
in nulliparous women, 0.46 in obese women (whose bodymass index at induction initiation was ≥ 29 kg/m2), and 0.48
in women belonging to the BS1–3 class. The AOR of theneo-metoro group was 1.5, indicating insignificant advantage of neo-metoro use for reducing IDI.
In the 113 women who underwent LI via Schedule A,

N. Matsumoto, M. Osada, C. Matsumoto, Y. Gomi, S. Era, H. Udagawa, N. Suzuki, S. Takahashi

VDPS and VDF rates were 73% (83/113) and 88%(100/113), respectively. The VDPS rates were 69% and
82% in nulliparous and multiparous women (p = 0.18);
47%, 75%, 87%, and 75% in women belonging to the BS1–
3, BS4–5, BS6–7, BS8–10 classes (p = 0.025); and 78%
and 61% in the neo-metoro and mini-metoro groups, respectively (p = 0.095). The VDF rates were 82% and 100%in nulliparous and multiparous women (p = 0.004); 74%,
87%, 100%, and 100% in women belonging to the BS1–3,
BS4–5, BS6–7, BS8–10 classes, (p = 0.034); and 90% and
84% in the neo-metoro and mini-metoro groups, respectively (p = 0.34).
Discussion
In the present study, the authors evaluated and showed
the effectiveness and safety of methods using neo-metoro
or mini-metoro metreurynters in combination with subsequent oxytocin for LI. They also assessed the differences inparturition outcomes according to metreurynter type used at
induction initiation; neo-metoro use at induction initiation
might shorten IDI relative to mini-metoro use.
The overall VDF rates were 78% and 96% in nulliparousand multiparous women, respectively. In a previous study
at this institution (n = 315) [7], vaginal delivery rates inwomen with spontaneous labor onset were 91% and 97% innulliparous and multiparous women, respectively. Therefore, in nulliparous women who underwent LI, vaginal delivery rate was lower than in women who experienced
spontaneous labor onset (p < 0.01). In contrast, there wasno difference in vaginal delivery rates between multiparous
women who underwent LI and those who experienced
spontaneous labor onset. These results indicate that nulliparous women attempting LI have an approximately a twofold higher risk of cesarean delivery, consistent with otherstudies [8, 9].
Among the 113 women who underwent LI via Schedule
A of the two-day induction using neo-metoro or minimetoro prior to oxytocin, the VDF rates were 82% and100% in nulliparous and multiparous women, respectively.
Levy et al. [10] reported LI using a Foley balloon catheter
prior to oxytocin (n = 203) similarly to the present method.
In their study, vaginal delivery rates were 78% and 99% innulliparous and multiparous women, respectively. The authors of this study believe that their results showed similar
efficacy of the Lavy’s method.
With regards to parturition outcomes, the advantages ofeach metreurynter used at induction initiation were unclear.
However, both the overall AOR for VDPS and the AOR forachieving delivery per unit time in the 113 women via Schedule A of the two-day induction showed the tendency that theneo-metoro group tended to achieve earlier delivery than themini-metoro group. Kaplan-Meier analysis of nulliparous
women via Schedule A showed the same tendency. Therefore, for achieving early delivery, neo-metoro use might con-15

fer an advantage to mini-metoro use in induction initiation.
On the other hand, regarding VDF, no advantage of neometoro use over mini-metoro use was evident. In their randomized controlled study, Levy et al. [10] compared theefficacy of 30 and 80 ml balloons for cervical ripening during LI. In their study, no difference in cesarean rate was evident between the two volumes. However in their subgroup
analysis, in nulliparous women, delivery rate within 24 hours
was higher and IDI was shorter in the 80 ml than in the 30 mlgroup. Such tendencies were not observed in multiparous
women of their study. Therefore, the present authors consider
that neo-metoro (which has a larger balloon than mini-metro)
use at induction initiation will not affect cesarean rate; however, neo-metoro use at induction initiation, especially in nulliparous women, may shorten IDI relative to mini-metoro use.
In the present study, lower BS at induction initiation was
associated with lower VDPS and VDF rates and longer IDI.
Failed induction is recognized to occur more often in
women with an unfavorable cervix. In their prospective
study (n = 134), Reis et al. [11] suggested that unfavorable
cervical maturity (gauged by abbreviated BS) at admission
for LI correlates with not achieving delivery within 24
hours in nulliparous and multiparous women. In the present study, the authors showed that IDI in women with a BS
≤ 3 is longer than in women with a BS ≥ 6 (Table 3). In
their prospective study of 1,389 nulliparous women who
underwent spontaneous labor onset or LI, Vrouenraets etal. [8] suggested that a BS of ≤ 5 was a risk factor for cesarean delivery (AOR 2.3) compared with a BS of ≥ 6. In
the present study, the authors showed that in nulliparous
women who underwent LI, a BS of ≤ 5 was a risk factor
for failed induction (cesarean delivery), and that a BS of≤ 3 greatly increased this risk (Table 2).
No differences were apparent in adverse events rates between the metreurynter groups. Moreover, severe adverse
incidents were not observed in this study. Concerning methods of cervical ripening and LI, it is recognized that transcervical Foley balloon catheters have a lower risk ofuterine hyperstimulation but a slower effect on labor thanvaginal prostaglandins [2, 12]. The authors did not directly
examine rates of uterine hyperstimulation and non-reassuring fetal status in this study. Moreover, this study might
be too small to evaluate the adverse events that would occur
during labor and delivery. However, overall rates of thethree adverse events which were assessed in this study were
acceptably low and no differences in adverse events rates
with metreurynter usage were evident. Therefore, the authors do not believe that metreurynter use will raise therisks of adverse events. Actually, in their experience of LI,
severe incidents were rarely observed. In contrast, many
Japanese obstetricians recognize that metreurynter use cancause umbilical cord prolapse [4]. In a retrospective observational study of 766 term women with cephalic presentation who underwent LI, Hirashima et al. [5] showed that aspherical metreurynter inflated with 200 ml or more caused

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Labor induction using modified metreurynters plus oxytocin at an institution in Japan: a retrospective study

umbilical cord prolapse more often than 150 ml or less. In
their case-controlled study including 370 women with LI
using neo-metoro plus oxytocin, Maruyama et al. [6] reported a case with umbilical cord prolapse and subsequentneonatal death in the LI group. However, no well-designed
studies address whether transcervical balloon catheterscause umbilical cord prolapse. The present authors believethat balloon catheters, especially neo-metoro and minimetoro metreurynters which are used for balloons inflatedwith 100 ml or less, are sufficiently safe to mature thecervix and induce labor.
In this study, the authors presented an example of themethods for LI using modified metreurynters and oxytocin
in Japan. They hope this paper will form the basis of further
studies to solve the controversial and unsolved problems of
LI in Japan.
Acknowledgment
A summary of this study was presented at the 122nd Academic
Conference of the Kanto Society of Obstetrics and Gynecology,
Yokohama City, Kanagawa, Japan, 2011.

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Address reprint requests to:
N. MATSUMOTO, M.D.
3-2-7 Sakae, Honjo City
Saitama 367-0046 (Japan)
e-mail: research@matsumotoc.org