Fetal station based on the trapezoidal plane and assessment of head descent during instrumental delivery Hypertension Research In Pregnancy

Abstract

The precise reporting of fetal station is important in the decision-making regarding whether instrumental vaginal delivery or cesarean section should be performed. However, accurate evaluation of fetal station is difficult because it is defined on the basis of a hypothetical vertical midline to the ischial spines. Moreover, during delivery, the fetal head descends anteriorly into the pelvis along the pelvic axis and not in the vertical direction. DeLee's concept of fetal station, first reported in 1924, has been revised by taking into account the fetal head descent along the pelvic axis, and this concept has been in clinical use at the University of Tokyo Hospital since the 1970s. In this review, we assess the problems associated with conventional fetal station and explain the new concept of fetal station based on the trapezoidal plane and assessment of head descent upon instrumental delivery.

Full text

S. Takeda et al.
65Hypertens Res Pregnancy 2014; 2: 65–71
REVIEW
Fetal staon based on the
trapezoidal plane and assessment
of head descent during instrumental
delivery
Satoru Takeda, Jun Takeda, Taro Koshiishi, Shintaro Makino,
Katsuyuki Kinoshita
Department of Obstetrics and Gynecology, Juntendo University, Tokyo, Japan
Hypertension Research
In Pregnancy
Reprint request to:
Satoru Takeda, M.D., Ph.D.,
Department of Obstetrics and
Gynecology, Juntendo University,
2-1-1 Hongo, Bunkyo-ku, Tokyo,
113-8421, Japan.
E-mail: stakeda@juntendo.ac.jp
Key words:
assessment of head descent,
forceps delivery, instrumental
delivery, trapezoidal fetal staon
Received: November 5, 2014
Revised: November 25, 2014
Accepted: December 12, 2014
DOI:10.14390/jsshp.2.65
The precise reporng of fetal staon is important in the decision-making regarding whether instrumental vaginal
delivery or cesarean secon should be performed. However, accurate evaluaon of fetal staon is dicult because
it is dened on the basis of a hypothecal vercal midline to the ischial spines. Moreover, during delivery, the
fetal head descends anteriorly into the pelvis along the pelvic axis and not in the vercal direcon. DeLee’s
concept of fetal staon, rst reported in 1924, has been revised by taking into account the fetal head descent along
the pelvic axis, and this concept has been in clinical use at the University of Tokyo Hospital since the 1970s.
In this review, we assess the problems associated with convenonal fetal staon and explain the new concept of
fetal staon based on the trapezoidal plane and assessment of head descent upon instrumental delivery.
Introducon
The concept of fetal station was initially described by
DeLee1) in 1924 as the level of the presenting fetal
part in the birth canal in relation to the ischial spines1).
Later, in 1988, the American College of Obstetricians
and Gynecologists (ACOG) rst reported on a station
classication system wherein the pelvis above and below
the ischial spines is divided into fths.2,3) These divisions
are represented in centimeters; if the leading part of the
fetus descends at a level between the spines, the station is
designated as 0. Below the ischial spines, the presenting
fetal part passes stations + 1, + 2, + 3, + 4, and + 5 to
delivery4,5) (Figure 1).
In order to assess the progress of labor, proper
reporting of fetal station, as well as cervical dilation and
effacement, is essential. In addition, precise reporting
of fetal station is also important in the decision-making
regarding whether instrumental vaginal delivery or
cesarean section should be performed. However, accurate
evaluation of fetal station is difcult due to the facts that
it is dened on the basis of a hypothetical vertical midline
to the spines and that the ndings may vary between
individuals.6) Moreover, during delivery, the fetal head
descends anteriorly into the pelvis along the pelvic axis
and not in the vertical direction. For these reasons, the
denition of fetal station proposed by DeLee might not be
suitable for the precise and objective assessment of fetal
Figure 1. The concept of convenonal fetal staon.
DeLee’s staon is dened according to the levels of the
leading poron of the fetal head in cenmeters
horizontally at or below the levels of the maternal ischial
spines. However, the fetal head actually descends
anteriorly along the pelvic curve.
65
Hypertension Research in Pregnancy © 2014 Japan Society for the Study of Hypertension in Pregnancy

Fetal staon & head descent assessment
66 Hypertens Res Pregnancy 2014; 2: 65–71
engagement and descent. In fact, several studies have
shown that fetal head engagement and descent can be
assessed more objectively and measured more precisely
using ultrasonography or MRI compared to fetal station
evaluation.7– 15)
DeLee’s original concept of fetal station has been
revised by taking into account the fetal head descent
along the pelvic axis, and this concept has been in
clinical use at the University of Tokyo Hospital since the
1970s.16) Takeda and Kinoshita17) dened fetal station
based on the triangular or trapezoidal plane consisting
of the ischial spines and the lower edge of the pubic
symphysis along the pelvic axis as the t-station, and this
denition has been clinically used at Saitama Medical
Center and Juntendo University Hospital since 1985
and 2001, respectively. Unfortunately, to date, this new
Figure 2. New fetal staon based on the trapezoidal
plane (t-staon).
The denion of a new fetal staon along the pelvic axis
is based on the trapezoidal plane consisng of both
ischial spines and the lower edge of the pubic symphysis.
In the clinical seng, this t-staon would be more useful
for assessing the descent of the fetal head into the pelvic
cavity aer engagement.
(A) Oblique view
(B) Front-caudal view
Figure 3. Direcon of the fetal head descending
anteriorly along the pelvic axis.
This pelvic schema is copied as a precise reduced
drawing from radiographic pelvimetry. A top fetal head is
a real reduced-size fetal head but others are just moving
to each staon by itself.
The fetal leading head descended anteriorly along the
pelvic axis. With the descent of the head, a decrease in
the degree of obliteraon in the space between the
posterior surface of the pubic symphysis and the fetal
head is noted.
①The true obstetric conjugate
②The trapezoidal plane
concept of t-station has only been reported in textbooks
of obstetrics written in Japanese.16,17)
For this reason, in this brief review, we intend to
explain the concept of t-station, which represents a more
objective assessment metric for head descent.
Denion of fetal staon on the basis
of the ischial spines
DeLee dened fetal station as the vertical distance (in
centimeters) from the line between the ischial spines
(base level: 0) in Hodge’s parallel pelvic planes, to the
presenting part. The stations above the line between the
ischial spines are designated − 1 to − 5 and those below
the line, + 1 to + 5 (Figure 1). However, when the fetal
head is engaged and descends in the pelvis, the presenting
part passes more anteriorly compared to the vertical axis
on the line between the ischial spines, in accordance with
the obstetric pelvic axis. Thus, estimation of fetal head
①
②

S. Takeda et al.
67Hypertens Res Pregnancy 2014; 2: 65–71
station using this system does not accurately reect on
actual head descent and progression.
Denion of fetal staon on the basis
of the trapezoidal plane (t-staon)
A revised denition of fetal station (t-station) has been
previously proposed. This denition is based on the
trapezoidal plane, which consists of both the ischial
spines and the lower edge of the pubic symphysis along
the pelvic axis. The t-station is dened as the shortest
distance from the trapezoidal plane (base level: 0) to
the presenting part (Figure 2). This revised denition
is considered much easier to understand than DeLee’s
original denition and can objectively estimate how the
fetal head descends anteriorly along the pelvic axis as
a curve (Figure 3). In fact, it is possible to measure the
t-station between the trapezoidal plane and the leading
portion of the fetal head with the breadth of each nger
and the length of the space between the index nger
and the middle nger. The distance from the upper edge
of the index nger can directly measure t-station when
the index nger is placed on the ischial spine, the upper
edge of the index nger is set at the lower edge of the
pubic conjugate during vaginal examination, and the
descending fetal head is touched using the middle nger
while bent (Figure 4). Thus, the descent of the fetal
head can be objectively assessed, thereby aiding in the
decision regarding whether instrumental vaginal delivery
or cesarean delivery is most suitable.
Esmaon of the site of the largest
fetal head circumference in the pelvis
Appropriate vaginal examination is critical not only
for observing the fetal position, exion, and molding,
but also for the accurate diagnosis of fetal descent
into the pelvis in terms of fetal station. Moreover, the
ndings in each case must be explained and shared
at clinical conferences for obstetricians involved in
training and who practice instrumental delivery. In
particular, it is important to estimate the site of the
largest circumference, which is usually identical to the
site of the suboccipitobregmatic diameter of the fetal
Figure 4. Vaginal examinaon to measure fetal descent.
(A) Lateral view of the pelvis.
In vaginal examinaon, the physician touches the lateral ischial spine with the index nger-p and places the upper
edge of the index nger close to the lower edge of the pubic symphysis. Then, the bent middle nger can feel the
leading poron of the fetal head. If the fetal head is oang, this bent middle nger cannot touch the fetal head.
However, depending on the leading poron descending into the pelvic cavity, it can be touched by the bent middle
nger. Consequently, the t-staon can be recognized by measuring the distances between the upper edge of the index
nger and the upper edge of the bent middle nger. For this, the physician needs to know the breadth of each nger
at the distal or proximal interphalangeal joint and at the metacarpophalangeal joint as well as the length of the
maximum space between the index and middle ngers.
This denion of the t-staon is expected to be especially useful for instrumental delivery because of an objecvely
measurable staon, being more objecve and accurate than the DeLee’s staon.
(B) Cephalad view of the pelvis.
It was possible to measure and esmate the t-staon in cenmeters, according to the length of the maximum space
between the index and middle ngers.

Fetal staon & head descent assessment
68 Hypertens Res Pregnancy 2014; 2: 65–71
head at occiput anterior presentation (Figure 5), for the
decision-making regarding whether instrumental vaginal
delivery or cesarean delivery is indicated for cases of
non-reassuring fetal status, arrested labor, etc. The site of
the largest fetal head circumference, which corresponds
to the site of the occipitofrontal diameter at the occiput
posterior position, needs to be carefully estimated. The
distance between the leading portion and the site of
the largest circumference is obviously more in cases in
which the infant is large, the head has extreme molding
at occiput posterior presentation (Figure 6), or there is a
large caput succedaneum, as well as in cases of prolonged
labor, than in cases of normal smooth delivery and when
the head is small or has less molding. In the former
cases, it is extremely difcult to estimate the site of the
Figure 5. Relaon between t-staon (0, + 2, and + 4), the palpable
range of the posterior surface of the symphysis, and the site for the
largest fetal head circumference ( ).
(A) The fetal head at t-staon ± 0.
The site of the largest fetal head circumference usually descends
through the true obstetric conjugate at t-staon ± 0. All posterior
surfaces of the pubic symphysis are palpable.
(B) The fetal head at t-staon + 2.
The site of the largest fetal head circumference is usually located above
the line drawn between the lower edge of the S2 vertebra and the
posterior surface of the pubic symphysis. At this point, two-thirds to
half of the posterior surface of the pubic symphysis are palpable. There
is a wide, empty space in front of the sacrum.
(C) The fetal head at t-staon + 4.
The site of the largest fetal head circumference is typically located
below the line drawn between the lower edge of S2 and the posterior
surface of the pubic symphysis. The posterior surface of the pubic
symphysis can be barely palpated. There is no space between the fetal
head and the sacrum.
largest fetal head circumference and to safely perform
instrumental delivery if only fetal station and cervical
are dilatation.
To ensure safe and reliable vacuum extraction or
forceps delivery, it is necessary to understand the
positional relationship between the fetal head and pelvis.
Thus, assessments of fetal station, the site of the largest
fetal head circumference, rotation of the fetal head, and
the degrees of head exion are all important for safe
instrumental delivery. Another important parameter is
the palpable range of the posterior surface of the pubic
symphysis, expressed as all, 2/3rd, 1 / 2nd, 1/3rd, or none.
As the fetal head descends into the pelvis, the degree of
obliteration of the space between the posterior surface
of the pubic symphysis and the fetal head decreases;

S. Takeda et al.
69Hypertens Res Pregnancy 2014; 2: 65–71
Figure 6. The higher site of the largest fetal head circumference in the occiput-posterior presentaon compared with
that in the occiput-anterior presentaon.
The site of the largest fetal head circumference ( ) may be aected by several factors such as the size of the fetal
head, size of the caput succedaneum, abnormal rotaon, presentaon and atude, the degree of head extension, and
asynclism. Therefore, the site of the largest fetal head circumference is dierent in each case, even with the same
fetal staon. Both cases show the same t-staon + 3.
(A) The occiput-anterior presentaon.
In this presentaon, the largest fetal head circumference is equal to the suboccipitobregmac circumference.
(B) The occiput-posterior presentaon.
In this presentaon, the largest fetal head circumference is equal to the occipitofrontal circumference. In this abnormal
presentaon, the site of the largest head circumference is located much higher than that in the occiput-anterior
presentaon.
Table 1. ACOG classicaon of forceps deliveries according to the DeLee’s staon compared with that according
to t-staon
Types of Forceps
Deliveries
ACOG, 2007 T-Staon
The DeLee’s
Staon Rotaon T-Staon The palpable range
of the symphysis
The anterior space
of the sacrum
Midforceps +1 +1 all
Low forceps +2 >45°
+2
(high level) 2/3-1/2 wide
+3
(low level) 1/2-1/3 narrow
Outlet forceps
not
exceeding
45°
+4
(outlet) <1/ 3-none none
ACOG, the American College of Obstetricians and Gynecologists.
consequently, the palpable range of the posterior surface
of the symphysis is gradually reduced (Figure 5, Table
1). During forceps or vacuum delivery, to determine the
angle of the posterior surface of the pubic symphysis by
vaginal examination, the fetal head must be pulled in the
correct direction. Additionally, to assess the positional
relationship between the head and the pelvic oor, such
as the anterior space of the sacrum, it is important to

Fetal staon & head descent assessment
70 Hypertens Res Pregnancy 2014; 2: 65–71
determine the degree of fetal head descent, expressed as
wide, narrow, or none (Table 1).
Criteria for types of forceps deliveries
The ACOG criteria used to determine the type of forceps
suitable for delivery are essentially based on the four-
level pelvic plane. Accordingly, midforceps delivery is
used for fetal head engagement above stations + 2 cm,
while low forceps delivery is used when the leading
portion of the fetal head is at station ≥ +2 cm but not
on the pelvic oor. Outlet forceps delivery is employed
when the fetal scalp is visible at the introitus without
separating the labia, the fetal head has reached the pelvic
oor, the sagittal suture is in the anteroposterior or right
or left occiput anterior or posterior position, the fetal head
is at or on the perineum, and the rotation does not exceed
45° (Table 1).
On the other hand, t-stations − 1 cm to 0 cm, dened
according to the trapezoidal plane, refer to when the
site of the largest fetal head circumference is located at
the pelvic inlet; whereas t-stations 0 cm to + 1 cm, + 2
cm to + 3 cm, and + 4 cm to + 5 cm refer to when this
site is located at the upper middle pelvis (midforceps
delivery), the lower middle pelvis to the low pelvis
(low forceps), and when the head is in the low outlet
pelvis (outlet forceps delivery), respectively (Table 1).
Correct estimation of the site of the maximum fetal head
circumference is important for instrumental delivery,
such as forceps delivery, which depends on the size of
the fetal head and the axis of rotation of the fetus. The
positions seem to be slightly higher for t-stations 0 cm
to + 2 cm than for the corresponding DeLee stations,
although this difference is negligible in the case of the
lower t-stations.
At Juntendo University, the prerequisite for successful
application of forceps delivery is t-station ≥+3 cm. Only
an expert may attempt forceps delivery at t-station ≥ +2
cm, and preparation for a cesarean delivery must be made
alongside. Several factors may affect the estimation of
the site of the largest fetal head circumference, such as
the size of the fetal head, size of the caput succedaneum,
abnormal rotation, presentation and attitude, degree of
head extension, and asynclitism. Full attention should be
paid to cases in which the fetus is in the frontoanterior
position because the fetal head may wrongfully appear
descended, and the site of the largest head circumference
is at a high position (Figure 6). This kind of misdiagnosis
may require traction from a high level and can cause
unexpected problems.
Conclusion
Accurate estimation of fetal station using vaginal
examination is extremely important in cases in which
forceps delivery is being considered, owing to the fact
that this type of delivery cannot be applied on a trial
basis. Therefore, the focus of training for forceps delivery
should be on understanding the fetal head station at
which safe and reliable delivery can be ensured. We
believe that to effectively obtain objective and practical
ndings from internal examination, the presented concept
of evaluating fetal head descent on the basis of the
pelvic axis is better than the conventional station system
proposed by DeLee. Further studies should be performed
to demonstrate clinical evaluations and usefulness
of t-station, compared with outcome of instrumental
delivery by the conventional station.
Conict of interest
None.
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