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490 British Journal of Midwifery • July 2015 • Vol 23, No 7
research
© 2015 MA Healthcare Ltd
Female pelvic shape: Distinct types
or nebulous cloud?
For well over 50 years, students of midwifery,
obstetrics, gynaecology and related
professions have been taught the Caldwell-
Moloy classification of the female pelvis. While
recognising variation and mixed types, the
current e-book of Mayes’ Midwifery (MacDonald
and Magill-Cuerden, 2011), is typical in its frank
reporting and use of that system. ‘Although there
are four recognised pelvic categories (Caldwell
et al, 1940: Table 24.1), variations within these
categories can occur. Some women may have mixed
features, such as a gynaecoid posterior pelvis and
android fore-pelvis’ (Burden and Simons, 2011:
286). The 10th edition of Clinical Obstetrics also
gives Caldwell-Maloy classification system similar
prominence (Gopalan and Jain, 2005).
The basic ‘four types of pelvis’ categorisation
persists to this day and is evidenced by the fact
that it is taught in many universities and colleges
globally and regularly cited in university-level
midwifery course compulsory texts, such as Mayes
Midwifery (MacDonald and Magill-Cuerden,
2011) and Myles’ Textbook for Midwives (Fraser
and Cooper, 2009) and referred to in recent
studies on female pelvic shape variation (e.g.
Hashemi et al, 2010). The four types of pelvis
categorisation persists even though the initial
simple classification was subsequently extended
by Caldwell et al (1940) to include 14 sub-types,
and that it has also been criticised, for example,
for being overtly racist (Geller and Stockett, 2007).
A recent study of 172 women in a Latvian
population reported a cluster analysis of four
measured diameters of the pelvis; however,
these did not mirror the Caldwell-Moloy (1933)
classification. Using SSPS, the researchers found
three clusters based on an analysis of two linear
measures, the anterio-posterior diameter and
lateral diameters (Kolesova and Vetra, 2012). In
contrast, the 3-D GM analysis of the population
reported in this Western Australia (WA) study
found no such clustering.
This WA study is part of a larger project
analysing pelvic shape for sex estimation in a
forensic context (Franklin et al, 2012a; 2014).
These methods were applied to investigate the
long standing pelvic classification of Caldwell
et al (1940), in order to re-evaluate its biological
foundations. The aim of the study was to elucidate
whether these classic female pelvic types are
an accurate reflection of the real morphometric
variation present in the female human pelvis.
What is the basic classification?
The pelvic shape types in the Caldwell–Moloy
classification will now be defined. These are more
fully described in Caldwell et al (1940).
The four major types are: gynaecoid, android,
anthropoid, and platypelloid (Figure 1). The
gynaecoid (Greek: gyne + eidos = ‘woman type’)
form is the type allocated to the ‘normal’ female
form and has a round or slightly transversally
oval shaped pelvic brim. It has a wide sub-pubic
arch and the sacrum is inclined posteriorly. The
android (Greek: andros + eidos = ‘man type’) form
has the ‘classic’ male, ‘pear shaped’ brim with the
widest transverse diameter at the brim, closer to
the sacrum than the pubis. The sub-pubic arch is
narrow, the sacrum is inclined anteriorly and the
cavity is funnel shaped with prominent ischial
Abstract
The objective of this study was to re-evaluate the Caldwell-Moloy (1933)
classification of female pelvic shape, which has been traditionally, and still
is currently, taught to students of midwifery and medicine. Using modern
pelvimetric methodologies and geometric morphometric (GM) analysis
techniques, we aim to elucidate whether these classic female pelvic types
are an accurate reflection of the real morphometric variation present in
the female human pelvis.
GM analysis was carried out on sets of pelvic landmarks from scans
of women living in a contemporary Western Australian population. Sixty-
four anonymous female multi-detector computer tomography (MDCT)
scans were used for most of the study and 51 male scans were also
examined for comparison.
Principle component analysis (PCA) found that there was no obvious
clustering into the four distinct types of pelvis (gynaecoid, anthropoid,
android and platypelloid) in the Caldwell-Moloy classification, but rather
an amorphous, cloudy continuum of shape variation.
Until more data is collected to confirm or deny the statistical
significance of this shape variation, it is recommended that teachers
and authors of midwifery, obstetrics and gynaecological texts be more
cautious about continuing to promote the Caldwell-Moloy classification,
as our results show no support for the long taught ‘four types’ of pelvis.
Keywords: Female pelvic shape, Caldwell and Moloy, Geometric
morphometric analysis
Algis Kuliukas
PhD Candidate
University of Western
Australia
Lesley Kuliukas
Midwifery Lecturer
Curtin University Western
Australia
Daniel Franklin
Director and Associate
Professor
University of Western
Australia
Ambika Flavel
Research Associate
University of Western
Australia
491
British Journal of Midwifery • July 2015 • Vol 23, No 7
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Following this, each anterior type could
theoretically be combined with any of the four
posterior types, yielding 16 possible types and,
assuming the same number of permutations could
exist for both the inferior and posterior aspects of
the pelvis, there could be as many as 256 subtypes
in total.
However, Caldwell et al (1940) chose not to
include a distinction between inferior and superior
aspects in their subtype classification and decided
that it was impossible for the anthropoid segment
spines. Anthropoid, despite its name (Greek:
anthropos = ‘human’), denotes a more ape-like
shape, where the anterior-posterior (AP) diameter
at the brim is significantly larger than the lateral,
giving a long narrow oval shape. Finally, the
Platypelloid (Greek: platys + eidos = ‘flat type’)
form is where the lateral diameter at the brim
is significantly larger than the AP, giving a flat
or transversally oval form (Caldwell et al, 1940;
Burden and Simons, 2011).
Caldwell and Moloy (1938) found that the
gynaecoid form was the most common among
women, with around 42% of the populations
studied having this type. In their study, ‘White’
and ‘African American’ females were published
separately—in Figure 1, they are combined.
How was the basic classification
enhanced?
Caldwell and Moloy (1938) and Caldwell et al
(1940) extended their classification. In addition
to the four ‘classic’ or ‘pure’ types, it was decided
that combinations (‘mixed’ types) were not only
possible, but actually more likely. They describe
‘departures’ where the classic form at the pelvic
inlet differs from that below it. For example, a
classic gynaecoid laterally oval pelvic brim shape,
may be reclassified ‘mixed’ if they also exhibit a
narrow sub-pubic arch, an anteriorly tilted sacrum
or narrower sciatic notch (Caldwell et al, 1940).
Other ‘mixed’ types were also reported where the
anterior section of the true pelvis differs from the
posterior segment. These became the basis for the
enhanced classification, where the parent type of
the posterior segment is combined with the parent
type of the anterior—e.g. a pelvis with a male-like
sacrum inclination and diminished sciatic notch
but with a wide sub-pubic angle would be termed
‘android-gynaecoid’ (Table 1).
Figure 1. The basic Caldwell-Moloy classification and reported prevalence (adapted
from Caldwell et al, 1940, data from US population. Black and White populations were
reported originally, combined here)
Table 1. Caldwell-Moloy subtypes, comments and numbering system
Posterior segment type
Gynaecoid Anthropoid Android Platypelloid
Anterior segment type
Gynaecoid True gynaecoid
(4, normal female pelvis)
Anthropoid–gynaecoid
(2)
Android–gynaecoid
(10)
Flat–gynaecoid
(13)
Anthropoid Gynaecoid-anthropoid
(5)
True anthropoid
(1, transversally contracted
type)
Android-anthropoid
(9)
Flat–anthropoid
(not included)
Android Gynaecoid-android
(6, gynaecoid with
narrow front pelvis)
Anthropoid-android
(3, anthropoid with narrow
fore pelvis)
True android
(8, masculine type,
funnel type)
Flat–android
(14)
Platypelloid Gynaecoid-flat
(7)
Anthropoid-flat
(not included)
Android-flat
(11)
True platypelloid
(12)
From: Caldwell et al (1940)
492 British Journal of Midwifery • July 2015 • Vol 23, No 7
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position equally and the flat sides and large anterior
posterior diameter to the outlet allows the fetus to
descend without rotation; therefore, the baby may
be born in a persistent occipito-posterior position
(Burden and Simons, 2011). A platypelloid pelvis
would encourage engagement in the occipito-lateral
position and is more likely to cause asynclitism in
labour as one parietal bone enters the pelvis before
the other. The android and platypelloid pelvis types
are most likely to cause obstruction (Burden and
Simons, 2011).
The clinical rationale for categorising pelves was
to be able to predict these various mechanisms
of labour and the possibility of cephalopelvic
disproportion (Caldwell and Moloy, 1933). However,
over time, it became apparent that theoretical
pelvic shapes do not necessarily determine labour
outcomes. It is virtually impossible to predict
birth outcomes before labour even starts (Hanzal
et al, 1993; Spörri et al, 2002). The need for
pelvic categorisation was therefore questioned
from a practical obstetric point of view, yet the
classification is still described in current textbooks
for midwives and obstetricians (Gopalan, 2005;
Fraser and Cooper, 2009; MacDonald and Magill-
Cuerden, 2011).
Potential racist criticisms of the
Caldwell-Moloy classification
Caldwell and Moloy wrote many statements that can
potentially be perceived as racist today. For example,
the populations that the authors linked more closely
to the ‘anthropoid’ pelvis are characterised as
‘primitive races’ (Caldwell and Moloy, 1938: 5). Since
then a number of fossil pelves (e.g. Australopithecus
afarensis; Tague and Lovejoy, 1986) have been
found, attributed to putative hominin ancestors,
which are distinctly platypelloid in shape, a form
that Caldwell and Moloy described as ultra-human
and not intermediate between humans and the
great apes in shape (Australopithecus africanus;
Zuckerman et al, 1973).
New morphological approaches
Although some of the X-ray imaging methods
used by Caldwell and Moloy to identify the shapes
of female pelves were innovative in their time,
the original basis of the ‘four type’ classification
was simply observational. Various measurements
taken of ratios and angles of selected pelvic
landmarks were used to help classify pelves, but
the decision was always, in the end, one based on
a subjective assessment.
More sophisticated techniques of shape analysis
are available today. For example, landmark-based
GM methods have been successfully employed
to combine with a platypelloid one in either of the
two combinations (Caldwell et al, 1940), resulting
in just 14 subtypes as outlined in Table 1.
It was proposed that ‘borderline types’ of pelvis
can be described by using these combinations
(Caldwell and Moloy, 1938), but there appears to
be little consideration that there may be a smooth
continuum of shapes from one type to the next,
within each segment type itself.
Sixty years of the Caldwell-Moloy
classification in midwifery,
obstetrics and gynaecology
Exactly when the Caldwell-Moloy classification of
the female pelvis became widely adopted as a
standard in the teaching of midwifery and medicine
is open to debate, but there seems little doubt that
by the 1950s it was already well established. Medical
and midwifery textbooks in the 1970s and 1980s
continued to categorise pelves in that way (Beischer
and Mackay, 1979; Bennett et al, 1989) and suggested
that the type of pelvis determined the passage of
the fetus during birth: ‘These differences in pelvic
shape are of more than radiological interest, since
they determine, in large measure, the mechanism
which is adopted by the fetus in passing through the
birth canal.’ (Beischer and Mackay 1979: 23). This
general assumption associated different types of
pelves with different mechanisms of labour because
of the differences within the true pelvis. This is
important because as Burden and Simons (2011:
286), in MacDonald and Magill-Cuerden point out,
‘the most important factor is the true pelvic space
available for the fetus to descend and emerge from
the pelvis during labour.’
The ideal pelvis for childbirth allows the fetus to
engage in the transverse position due to the wide
transverse diameter at the brim, rotate mid-cavity
at the level of the pelvic floor as this plane of the
pelvis is circular and then birth in an occipito-
anterior position taking advantage of the long
anterio-posterior diameter at the pelvic outlet.
Pelves of different shapes are considered to affect
the labour in different ways. The android pelvis,
because of having more space posteriorly at the
brim, is considered to increase the likelihood of
an occipito-posterior engagement of the head
(Burden and Simons, 2011). The fetus may remain
posterior through labour or may become impinged
on the prominent diameter between the ischial
spines ischial spines during the rotation mid-cavity,
causing a deep transverse arrest necessitating birth
by rotational forceps or caesarean section. The
anthropoid pelvis, with its long anterio-posterior
oval shaped brim causes the fetus to engage in
either the occipito-anterior or occipito-posterior
493
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for several years in various studies in physical
anthropology, palaeoanthropology and forensic
science, and have been shown to elucidate novel
morphometric features not readily described using
simple linear measurements (Franklin et al, 2014).
Rather than simply using ‘traditional’ ratios (e.g.
lateral pelvic diameter divided by anterio-posterio
pelvic diameter) and angles (e.g. sub-pubic)
to attempt to classify pelves, large groups
of homologous landmarks can be captured as
discrete 3-D (x, y, z) co-ordinates, stored, rendered
graphically and then statistically analysed, all
using sophisticated modern computer software.
Baab et al (2012) summarise these GM
techniques and some of their recent uses in
physical anthropology. They describe how, in
theory, GM methods mathematically describe the
whole shape of the specimens of interest so that
they may be evaluated objectively and statistically.
Their paper focuses on studies of the human skull,
but the greater shape variation in the human pelvis
(especially between the sexes) supports that these
techniques should yield potentially even more
valuable and clinically useful insights, as we hope
to show here.
The uncertainty surrounding the Caldwell
and Moloy classification provides an excellent
opportunity for the application of these methods.
In particular, the true pelvis, including features of
the pelvic brim, such as the sacral promontory;
mid-cavity, such as the subpubic arch and outlet,
such as the ischial spines, can be statistically
quantified using a relatively novel technique
in GM—semi-landmarks. Even the shape and
curvature of featureless regions of bone can be
traced using homologous sets of 3-D landmarks.
Following in the footsteps of Caldwell and
Moloy’s pioneering research, this study is also
based on the analysis of a living population of
females (and, for comparison, some males) taken
from a contemporary WA population, rather
than temporally and geographically removed
skeletal collections. Rather than using the fixed,
two-dimensional X-ray images that were taken
by Caldwell et al (1940) in postnatal women,
the present study analyses dynamic, 3-D images
generated from computer tomographic (CT) scans
using the image rendering program, Osirix™.
Hypothesis
If the Caldwell and Moloy Classification is
morphologically accurate, there should be
discernible clustering patterns visible when 3-D
GM data of female pelves are statistically analysed
in Morphologika. Specifically, there should be
four significant groups corresponding to their
four ‘types’ of pelvis when individual morphs are
plotted against the major principal components.
Methods:
Computer tomographic scans as a source
for pelvimetric population data
The Centre for Forensic Science at the University
of Western Australia (UWA) has been developing
techniques for acquiring 3-D landmarks in CT
scanned images. These techniques have been
applied in studies of various parts of skeletal
anatomy and results have been verified against
traditional measurement methods (Franklin et al,
2012a; 2012b; 2014).
CT scans are specifically well-suited to skeletal
biological research as the post-cranial skeleton
can be filtered out from the rest of the scanned
tissue with a high degree of resolution. The scans
assessed typically have a 1 mm slice thickness, the
visualisation of the regional skeletal anatomy is
therefore performed at a much higher level than
that of the X-rays used by Caldwell et al (1940).
Sample demographics
This study quantifies measurements from CT
scans from 64 women currently residing in WA.
A sample of individuals was randomly selected
from the WA Department of Health (DH)
Picture Archiving and Communication System
(PACS) database, and consists of adult patients
presenting at various hospitals during 2010–2011.
The scans were anonymous when received by
the authors with only sex and age data retained.
The mean age of the males was 47 years (range
22–63), and 44 (range 18–63) for the females.
Sample demographics are described in Table 2.
While undoubtedly comprising various ethnic
backgrounds characterised in the general WA
population, the sample is predominantly
Caucasian which typified the WA population.
Only individuals without obvious congenital or
acquired pelvic pathology were included.
Ethical approval
Ethical research approval was granted by the
Human Research Ethics Committee of the
University of Western Australia (RA/4/1/4362).
Landmark schema
Pelvic shapes were recorded using a defined set of
77 landmarks designed to represent homologous
bony features, these are used to generate semi-
landmarks that trace a further 150 relative smooth
and featureless curves and surfaces, making 228
landmarks in total. In addition, to point type
landmarks, taken from relatively easily-identifiable
494 British Journal of Midwifery • July 2015 • Vol 23, No 7
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These data are then downloaded and processed in a
custom-built database package.
3-D GM analysis with Morphologika
Prior to analysis, the landmarks obtained for
each specimen needed to be put into a common
co-ordinate system where all morph entities were
standardised for size and orientation leaving
relative variation purely to shape. Then, a PCA was
performed, which determines the key principal
components, which discriminate the pelves on
shape. Morphologika can then plot the individual
morphs against two axes, representing two principal
components, at a time. Using this method, most
of the shape variation in a set of morphs can be
visualised and explored. Morphologika displays
the landmarks with, or without, a wireframe to link
them (Figure 3) allowing them to be rotated and the
shape variation to be explored against the principal
components being analysed.
Results:
Anterio-posterior/lateral diameter ratio
cluster analysis
Following the Kolesova and Vetra (2012) study,
which found some clustering in their analysis of
anterio-posterior/lateral diameter ratios, these
measures and their ratios were extracted from this
data set for analysis. Generally, the range of ratios
ran smoothly from rather platypelloid (anterio-
posterior 83.4% of lateral) to rather anthropoid
(anterio-posterior 113% of lateral) in 63 of the 64
individuals. With the exception of one woman that
was markedly anthropoid at anterio-posterior 146%
of lateral, there were no significant clusters of data.
Female only
The PCA of the 64 female pelves revealed that
13% of the variation was accounted for by the first
principal component, 47% by the first six. No
obvious clustering patterns were visible in any
of the 15 pairwise comparisons of the top four
principal components (Figures 4 and 5).
Subjective assessment of female-only data
From the female-only data there is no obvious
clustering of points into four distinct shape
types so it was decided to add to the dataset
one of the four ‘type specimen’ in the Caldwell-
Moloy specification—the android (or ‘male’)
type. According to the traditional classification,
approximately 25% of female pelves should fall into
this category.
Males included
The sample was then analysed with male pelves
Table 2. Demographics
of participants
Age range Females Males
18–20 2 0
21–30 7 6
31–40 10 7
41–50 24 19
51–60 18 17
61–70 3 2
Total 64 51
Figure 2. Screen shot demonstrating semi-landmark
placement in OsirixTM
points on the pelvis, the shape of a series of
significant curvatures of bone, that are relatively
featureless, were also captured for analysis via
semi-landmarks.
Each semi-landmark set is defined by a point
type landmark to demarcate its start and end point
as well as the number of landmarks to be generated
along the curvature of bone (Figure 2). The semi-
landmark co-ordinates were generated by an
in-house developed database application, MorphDb.
Osirix™ data capture
Data are collected using the open source DICOM
(digital imaging and communications in medicine)
software package, Osirix™. The software allows the
DICOM file data to be filtered by the density of the
voxels (3-D pixels) so that only skeletal structures
are visible. The objects are thus rendered as 3-D
objects that can be rotated and zoomed in or out.
Osirix™ allows the placement of landmarks onto
the surface of the rendered image, each of which
captures the (x, y, z) co-ordinates of the point.
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included. If the Caldwell-Moloy android type of
female pelvis was found among this population, one
would expect to see some overlap with, or at least
a distinct grouping close to, the male distribution.
As shown in Figure 6, female and male and
pelvis shapes do not overlap at all when compared
by the most discriminatory principle component—
PC1, and there is no distinct group of females that
appears in close proximity to the male group.
Discussion
Historically, students of midwifery and medicine
have been taught that women will have either a
gynaecoid, android, anthropoid or platypelloid
pelvis. The results of this WA study did not reflect
the current taught concept that there are four
distinct type of pelvis. Rather than falling into
specific categories, the pelves formed a nebulous
cloud of variation. Furthermore, the concept of
25% of women having an android, or male shaped,
pelvis was refuted. In this study, the analysis of
both male and female pelves clearly demonstrated
two distinct groups with no overlap, suggesting
that the android shape occurs in women very
rarely, if at all.
The limitation of this study was that the sample
size was small and geographically specific. A
greatly expanded study, in terms of number and
geographical origin, would be required to address
this issue.
Conclusion
This study is preliminary in nature but appears
to provide sufficient evidence to cast doubts on
the Caldwell et al classification. These findings
suggest that it would be worthwhile to reconsider
the traditional teaching of midwifery, medical
and related professions that there are four distinct
types of female pelvis and, instead, encourage
Figure 3. Pelvis using semi-landmarks rendered without (and with) a wireframe
Table 3. Principal components
analysis of female pelves
Principal
component (PC)
Individual Cumulative
PC-1 0.129 0.129
PC-2 0.104 0.232
PC-3 0.083 0.315
PC-4 0.057 0.372
PC-5 0.054 0.426
PC-6 0.046 0.471
Figure 4. PC-1 x PC-2 (Female only)
(Pelvic images give indication of the shape
differences at the extremes of the principle
components)
the concept that the shape variation is simply
characterised by a cloudy continuum.
The key argument of this paper is to question
the traditional midwifery teaching that a woman’s
pelvis can be pigeon-holed, by its shape, into one
of four categories. Complex physical traits, such as
body height and the morphology of major skeletal
structures, are doubtless under the genetic control
of several alleles and many environmental and
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Key points
lWomen’s pelves may not be definable as one of the four types as
described by Caldwell and Moloy
lThe male pelvis (android) is distinct from the female pelvis
Figure 5. PC-3 x PC-4 (Female only)
(Pelvic images give indication of the shape
differences at the extremes of the principle
components)
Figure 6. PC-1 v PC-2 male and female. No
significant group of females overlap with, or even
cluster close to the male as one would predict if
there was an ‘android’ type of female pelvis
epigenetic factors. This type of control predicts
that any statistical analysis of related measures
will follow a smooth continuum in a normal
distribution as seen, for example, with body height.
Our results support the expectation that such a
smooth distribution of shapes exist in the pelvis.
Rather than trying to determine which distinct
type of pelvis a woman has, we argue that it is
more helpful to simply have an awareness that
pelvic shape has many components that may
affect childbirth, each of which can vary smoothly.
Certain phenomena, such as a high head at term
or deep transverse arrest, could be due to a more
exaggerated oval shaped brim or more prominent
ischial spines; to attempt to explain such events by
categorising the pelvis is not necessary and will not
affect the management of labour. BJM
Acknowledgements: The authors would like to thank over
100 anonymous people of Western Australia who agreed to
allow their MDCT scans to be used in scientific research.
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