Obstetrics and posture in pelvic anatomy

M. Maurice

Abitbol

Obstetrics and posture in pelvic anatomy

Department of Obstetrics and Gynecology, University Hospital, State C’niuersity ofNew York at Stotq Brook, Stonr Brook, New York; and Department of Obstetrics and Gjmecology, the Jamaica Hospital, 89th Avenue and Van Wyck Expressway, Jamaica, Near York, 11418. U.S.A.* *(Address

for reprint

To investigate the obstetric impact on the shape of the human pelvis; the pelvic dimensions of 500 female patients were compared with regard to the duration of their gestations, and to the weight of their newborns. It was demonstrated that there is no relationship between these variables. Similar observations were made on several mammals in which the dimensions of the pelvis are not influenced by the duration of gestation or fetal weight. although they are very responsive to sitting or erect posture. The pelvis in Australopithecus was shaped more to satisfy erect posture and bipedal locomotion than to allow increase in fetal size, which occurred much later, since the encephalization process was on the way only after the pelvis had taken more or less its present shape. Adjustments of the human female pelvis to the increased size of the fetal head are minor, compared with the adjustments to erect posture, because erect posture preceded encephalisation and was therefore first to make its demands on the pelvis. The modern human pelvis is certainly tight and frequently leads to difficult delivery. Socioeconomic factors such as diet, heavy physical work in adolescence, and early pregnancy also can be contributing factors to the multiple variety of shapes in the present female pelvis.

requests)

Received 8 October 1986 Revision received 11 March and accepted 2 July 1987 Publication date December

1987 1987

Keywords: Pelvic dimensions, duration of gestation, birth weight, erect posture, Australopithecus, Homo sapiens.

Journal

of Human Evolution (1987)

16, 243-235

Introduction Two groups of factors

have contributed

to the present

shape of the human pelvis: those

arising from the erect posture and those from the enlarged fetal head. The first group has been extensively

investigated

during the past century. The second group of factors also has

been mentioned by different authors. From a review of the literature, it is sometimes difficult to decide if a specific pelvic change is ofpostural or ofobstetric origin. For instance the backward obstetrical

tilt of the sacrum

by Schultz

is mentioned

as postural

by Reynolds

(193 1) and as

(1969);

wl‘d ening of the biacetabular diameter has been attributed to a better leverage for the back and abdominal muscles by some authors (Reynolds, 193 1) or to the passing fetal head by others (Robinson, 1972; Lovejoy et al., 1973). The lateral expansion of the sacrum was mentioned either as a stronger support for the erect trunk (Reynolds,

1931; Schultz,

1969; Robinson,

1972).

1969), or as necessary The elongation

better leverage for the adductor widening (Trinkaus, 1984). Investigating this problem

muscles

to widen the obstetrical

pelvis (Schultz,

of the pubic ramus also could be the result of a (Reynolds,

is of particular

193 1) ~or contribute

importance

because

to further pelvic

it is obvious

that the

female pelvis of modern Homo sapiens does not always respond to the actual obstetric requirements and in a certain percentage of the cases may even be unusually narrow (Caldwell & Moloy, 1933). Some anthropologists lesser pelvis play a fundamental role in determining the fetus at birth (Martin, elongated Neanderthal

have stated that the dimensions of the the duration ofgestation and the size of

1983). Trinkaus

in the Neanderthal

( 1984) noted that the pubic ramus was more than in Homo sapiens and concluded, therefore, that the

female carried a larger fetus in a pregnancy

lasting up to 11 months.

Martin

(1983) stated that the human fetus is born very prematurely, this happening solely because the human female pelvis would not allow a much larger fetus to go through. His implication is that if the pelvis was larger, the human gestation would last much longer and the human fetus at term would be much bigger. 0047-2484/87/030243

+ 13 $03.00/O

0

1987 Academic

Press

Limited

244

M. M. ABITBOL

The purpose of the present study is to investigate

in the human female the relationship

between the pelvic dimensions and duration of gestation or weight of the newborn. may help in determining what is postural and what is obstetric in origin.

Material The difficulty in studying

and methods

the evolution of the pelvis from the obstetric

one does not really know what to measure and what to compare measurements. each

plane

diameters

The human pelvis has three planes: the inlet, the midpelvis and the outlet; (TR) diameter. All these (AP) and a transverse

are significant

when one studies the anatomy

AP diameter

of the inlet or true conjugate,

female Homo sapiens. The diagonal measured

(2) The

point of view is that

among the different pelvic

has an anteroposterior

and the evolution

pelvis, but there are only three of them that are obstetrically

(1) The

This

conjugate

important

of the human

(Figure

1):

standard average 10.5 cm in modern (DC) is 1.5 cm larger and can be easily

by pelvic examination.

bispinal

(BS)

of th e midpelvis

measured accurately only by X-ray borderline, average, wide) is possible

varies

between

10 and 10.5 cm, but can be

pelvimetry. A clinical by pelvic examination.

evaluation

(narrow,

(3) The transverse diameter of the outlet or biischial (average 11 cm) can be measured clinically, but one can get a similar result by clinical evaluation of the subpubic angle (SA)

(narrow,

borderline,

average,

wide).

Figure 1. Schematic diagram showing the different diameters of the female pelvis. diameter of inlet. DC is diagonal conjugate. SS is bispinal diameter of mid pelvis.

The largest

part of the human

DA is anteroposterior SA is subpuhic angle.

fetus at term is the fetal head. It is spheroidal

in shape

with a diameter varying between 9 and 10 cm; it has to pass through all the aforementioned diameters to be delivered. The size of the fetal head is closely related to the total weight of the fetus, as demonstrated by sonography studies before birth (Parker et al., 1984) and by measurements on the newborn after birth (Hamill et al., 1979).

OBSTETRICS

The pelvic measurements were determined obstetric

department

December, patients

about

predisposing

THE

245

PELVIS

the weight of the fetus, and duration

women

of the Jamaica

1983. The who could

uncertain

as outlined,

in 500 pregnant

AND

as they were successively

of gestation

admitted

to the

(Q ueens, N.Y.) between 1 July and 27 types of patients were not included in this study: (1)

following

Hospital

not give an accurate

date of their last menstrual

the date of their conception;

to shorter gestation,

(2) patients

period

with pathological

such as uterine malformations,

or were

conditions

incompetent

cervix, etc.;

(3) patients who delivered a non-viable fetus before 28 weeks of gestation; (4) patients whose pregnancy was interrupted before the exact due date for different reasons, such as repeat

cesarean

section,

toxemia,

blood incompatabilities,

etc. Only patients

in this study. With the exceptions

who went

into spontaneous

labor were included

noted above, all

patients admitted

to the labor floor were included in this study until a total of 500 patients

was reached. In an attempt

to determine

the socioeconomic

background

were divided into two groups. The first group was composed were mostly black or Latin American; prenatal

they had fewer prenatal

of these 500 patients,

they

of 302 service patients;

they

visits and some of them no

care at all; they usually had a large family composed

those without young children doing heavy

manual

work, and commuted

dietary habits were judged was composed

of several young children;

usually worked up to an advanced extensively

to be inappropriate

of 198 private patients;

stage of their gestation,

via public

for a pregnant

transportation;

their

woman. The second group

they were usually white, they had complete prenatal

care, and the family was usually small; many of them did not work during pregnancy those who did had an easy desk job with easy commuting, weeks before the end of gestation; Eighteen

their diet was adequate

patients of east Asian background

in a separate

group because,

(1) The diagonal conjugate independent obstetricians

measured on pelvic in pelvic examinations

examination of pregnant

were divided

was considered

into four groups:

as the DC for that patient. values

below

considered to be “narrow”, between IO.5 and 11.5 were “borderline”, and 12.5 were “average”, and those above 12.5 were “wide.”

qualitative performed examiners (3) The

(BS) or distance between as “narrow”, “borderline”,

measurements

are routinely

determined

10.5 cm were between

11.5

the two ischial spines w-as “average”, or “wide”. These when a pelvic examination

is

on a pregnant patient as a part ofher clinical evaluation. Again, ifthe two disagreed in any case, the examination was repeated until they agreed.

subpubic

“narrow”,

by two patients;

until the results became closer, and the average

their two measurements

(2) The bispinal d’rameter qualitatively determined

difficulties

on the same patient differed by more than 0.5 cm, the two

repeated the examination

final values

were considered

obstetrical

for each patient:

(DC) was experienced

if their two measurements

These

Korea)

but not when they were in Asia. The spouses were all of

The following values were determined

value between

for their pregnancy.

Indochina,

as will be seen later, they presented

when giving birth in the U.S.A., East Asian origin.

examiners

(China,

and

and stopped working several

angle

“borderline”,

(SA)

was similarly

“average”,

determined

or “wide”.

by the two examiners

as

246

M.

M.

ABITBOL

was arrived at by combining the three previous (4) The total pelvic score (TPS) determinations in the following manner: for either DC, BS, or SA, the “narrow” qualification

was assigned

a “0” score, the “borderline”

a “2” score, and the “wide” a “3” score. For example, 10.5, a narrow bispinal,

and a narrow subpubic

as “1” score, the “average”

a small pelvis with a DC below

angle, was assigned a TPS of ““0”; a

wide pelvis with a DC above 12.5, a “wide” bispinal, was assigned

a.nd a “wide” subpubic

angle

a “9” score.

(5) The duration ofgestation by counting the number ofdays from the 14th day after the beginning of the last menses to delivery. (6) The weight of the newborn

was recorded

immediately

after birth.

Results Relationship

between pelvic dimensions and duration of gestation or weight of newborn

The 500 patients (DC}

in Table

were divided into four different groups of increasing

1, bispinal

total pelvic value (TPS)

diameter

in Table

regard to the average duration These

tables

measurements duration

show (DC,

diagonal

conjugate

angle (SA) in Table

3, and

4. Each DC, SA, BS, and TPS group was compared

ofgestation

there

is

or the average

increases,

with

and the average newborn weight for that group.

no

relationship

between

or weight of the newborn.

Table

Diagonal

conjugate

newborn

either

the

individual

on the one side, and the average weight on the other side. As any of these

one does not see any noticeable

gestation 1

2, subpubic

BS, SA) or the total score (TPS)

of gestation

pelvic dimensions

that

(BS) in Table

(DC) tabulated against duration Narrow

variation

in duration

of

of gestation and birth weight

(Below 10-5)

Borderline (10~~11~5)

Average (11~5-12~5)

Wide (Above 12.5)

Numberofpatients

47

109

288

56

Average duration gestation (wk)

39.2 t 3.1

38.7 + 2.4

39.4 rt: 1.3

39.0 + 3.3

3057 + 302

3290 t 177

3157 I 103

DC (cm)

Average Table 2

birth weight Bispinal

(g)

diaxneter (BS) tabulated against duration

BS Number

ofpatients

Average duration gestation (wk) Average

birth weight (g)

Narrow

Borderline

3102+314

of gestation and birth weight

Average

Wide

38

117

305

40

38-9 f 3.7

39.9 + 1.9

39.0 rt o-9

39-8 + 3.1

3156f

312

3149i

1.56

3195 F I21

3II2i297

Relationship between duration of gestation or weight of newborn and peluic dimensions The 500 patients after being divided into groups according to increasing gestational (Table

5) and increasing

birth weight

(Table

6), were compared

with regard

age

to pelvic

dimensions. These two tables took into consideration only the diagonal conjugate (DC) since this was the only mathematical value that was available, and the total pelvic score

OBSTETRICS

(TPS)

as outlined

each birth gestation increase

previously.

weight (Table

of gestation

Subpubic

values were averaged (Table

age and

the duration

6) and the pelvic dimensions,

of

since any

with any noticeable

Borderline

Wide

31

95

350

24

39.8 i 3%

39.0 f 1.8

39.1 f 1.9

40.4 + 3.8

3146 f 373

3097 F 203

3185 & 178

3241 t 307

Total pelvic score (TPS)

tabulated against duration

(cL1.5)

of patients

Average duration gestation (wk) Average birth weiqht CC)

94

361

19

40.0 + 3.7

39.4 f 1.9

39.2 + 2.1

39.4 + 3.7

3200 f 278

3166 L 108 conjugate

Duration 3638

3173-t202

gestation

38-40

40-42

11.0 * 0.7

11.0 ? 0.6

11.2 f 0.6

Total pelvic score

5.4 t I.0

5.1 ?I 1.1

5.3 + 0.3

5.2 f 0.3

1000

1000-2000

conjugate

2000-2500

370

(wk)

11.1 i- 0.8

Birth weight tabulated against diagonal

3108?

(DC) and total pelvic score (TPS)

Average diagonal conjugate (cm)

Birth weight (gm)

Wide (7.5-9)

Average (4.5-7.5)

26

of gestation tabulated diagonal

28-26

of gestation and birth weight

Borderline ( 1.5-4.5)

NLUTOW TPS

Table 5 Duration

AVlXige

of

Average birth weight (g)

Table 6

between

or birth weight was not associated

Narrow

ofpatients

Average duration gestation (wk)

Number

for each gestational

was noted

angle (SA) tabulated against duration of gestation and birth weight

SA

Table 4

247

PELVIS

in the pelvic dimensions.

Table 3

Number

THE

No relationship

5) or the birth weight

in duration

variation

These

in each group.

AND

42113

11.4*

Above 43

1.2

5.4 * 0

11.2 i 1.1 5.0 i 1.4

(DC) and total pelvic score (TPS)

2500-3000

3000-3500

1000

11.0?0.3

11.210.6

11.4 If- 1.3

Diagonal COnJUgatC

(Cm)

Total pelvic score

Relationship

11.1 i 1.1

11.2 t 1.0

11.4 f 0.3

5.3 f 1.3

5.4 + 1.0

5.3 rt 0.3

5.2 t 0.2

5.3 f 0.2

5.1 i 1.0

between duration of gestation and weight of the newborn

The 500 patients were divided into different groups depending on the age ofgestation at the time they went into labor. Each group of gestational age was compared with regard to the average weight of the newborn in that group (Table 7). This table shows that below 40 weeks of gestation, the newborn weight increased with gestational age; between 40 and 42 weeks, it does not appear that the fetus gains any weight; beyond 42 weeks, the fetus loses weight slowly during the 43rd week, and rapidly thereafter.

248

M. M.

Table 7

ABITBOL

Duration of gestation tabulated against birth weight Duration

Number patients

(wk)

28-36

3&38

38-40

40-42

42-43

Above 43

24

96

249

88

30

13

1502 f 150

2914 f. 182

3319 f 103

3370 + 180

3309 + 307

3152 j, 309

of

Birth weight (g)

Relationship weight

between socioeconomic background and pelvic dimensions, or duration ofgestation,

The two groups of private and service patients the average gestation,

pelvic dimensions and to the birth

between duration

as defined

weight

for each group

compared

with regard to

4 and 5, to the average (Table

8). There

duration

of

is no relationship

group than in the private one.

Table 8

Pelvic measurements

Type ofpatients Service (302)

(198)

The east Asian group Table 9 summarizes structure:

tabulated against socioeconomic

status

Diagonal conjugate (cm)

“Total pelvic score”

Duration of gestation (wk)

Birth weight

11.6kO.7

5.2 t 0.5

38.8 t 4.2

3081 IL 260

1 I.8 IL 0.7

5.3 + 0.4

39.7 + 4.1

3307 + 302

the experience

of 18 pregnant

born and reared in Asia ( 11 cases),

were generally

(9)

women of east Asian background.

Those

of small stature and had a small pelvic

four among them have already given birth in Asia while eating mostly a rice diet

and had no problem women became (Clark,

were separately

in Tables

or birth

the socioeconomic background and the pelvic dimensions. On the average, the of gestation is one week shorter and the birth weight is 10% lower in the low

socioeconomic

Private

ofgestation

1978)

section.

Women

pregnant

in the U.S.

a small baby (DOS Santos et al., 1979). But when these 11

in the U.S.,

and experienced

cesarean deliveries.

in delivering

pregnant

and ate a high protein diet, they had larger fetuses

difficulties

of east Asian

in giving

parents,

birth:

eight of them underwent

but born and reared

in the U.S.

a and

(seven cases), had normal size pelvis, normal size babies, and normal

The size of the pelvis has therefore

or the size of the fetus, and the latter depends

nothing to do with the duration essentially

of gestation

on a good diet.

It might have been useful to test the above results statistically using the ANOVA procedure: this would have been unsound however, because these data include both interval scores and measurements. As the above nine tables stand, the following conclusions can be drawn from their analysis:

(1) Among

modern humans there is no relationship between the dimensions of the pelvis and the duration of gestation or fetal size. If such a relationship existed, and in view of the tremendous variation in dimensions presented by the human female pelvis, one would expect a fair number of gestations of less than 9 months with a small pelvis, and gestations beyond 9 months with a large pelvis. This is not the case. The only connection between the pelvic size and the fetal size is that a normal fetus

OBSTETRICS

AND

THE

in a normal size pelvis will be delivered normally, will be delivered

(2)

happens

a small size fetus in a large pelvis

and a large size fetus in a small pelvis will be delivered with

easily,

dif~culty; but none of them will be delivered Pregnancy

earlier or later.

usually does not last beyond 9 months, regardless

pregnancy

249

PELVIS

and when this occurs

(and this

of pelvic size), the fetus does not benefit from it anyway.

If the

lasts more than two weeks beyond term, there is danger of a decreased

fetal size and even fetal death. (3) Good prenatal care and good diet help in bringing the pregnancy to an optimal duration and optimal fetal weight, but not a pregnancy beyond 9 months or an excessive

fetal weight.

All these findings

are confirmed

by similar

et al., 1976; Knox et al., 1979; NCHS,

Brenner

reports

in the literature

(Abitbol,

1964;

1983).

The East Asian patients

Table 9 Average diagonal conjugate

Xumber of patients

Subgroup’

(cm)

‘Total pelvic score’

Birth weight in Asia

Birth weight in U.S.A.

(9)

(9)

Number of cesarean sections in U.S.A.

1

7

11.1 f 1.1

5.0 Ik 0.8

3050 ZIG 290

1

2

7

10.1 f @9

2.9 + 0.3

2985 + 305

4

3

4

9.8 zk 0.8

2.4 f 0.2

3107 IL 318

4

2489Ik311

‘Subgroup: 1, patient is born in U.S.A. and delivered in U.S.A.; 2, patient U.S.A.; 3; patient is born in Asia, delivered first in Asia, then in U.S.A.

is born in Asia and delivered

in

Discussion If the pelvic dimensions reaching

an optimal

are the real factors

play no role, and good prenatal

duration

of gestation

that determine

gestational

very complex and not well understood depend

on the whole biology

especially

on the placenta

(Ramsey,

1962).

If fetal growth

duration

and fetal size? These factors are

(Fuchs et al., 1984; Lubchenco

of the pregnant

which

care plays a role only in terms of

and an optimal weight for the fetus, then what

woman,

acts as a transfer and duration

et al., 1963)) and they

on the uterine organ

of gestation

between

environment, mother

and

and fetus

are going to be reduced

or

increased, it will have to do more with these complex biological phenomena rather than with a small or a large pelvis. It appears that the placenta has a functional life span of 8.5 to 9.5 months,

beyond which it becomes

This limited life of the human placenta finishes at 9 months ofgestation, modern Homo sapiens, namely

non-functional

(Hertig,

1962; Abitbol

et al., 1976).

due to its rapid aging, which starts at 8 months and

leads us to the conclusion that hominids closely related to Homo sapiens neanderthalensis, must have had a similar

placenta. This argues against the hypothesis of Trinkaus (1983, 1984) whereby the Neanderthal female delivered a fully precocial neonate after a significantly longer gestation, and in favour of Rosenberg (1985) w h o considers the secondarily altricial Neanderthal neonate as to have been delivered after the same gestational length as modern hominids.

250

M.

M.

ABITBOL

The same observation about the lack of relationship between pelvic dimensions and fetal size can be observed in practically all mammalian species. In those non-primates where the neonate is altricial

(Schultz,

1969; Martin,

1983), the fetal head is very small compared

to

the dimensions of the maternal pelvis. The pregnant bitch for instance does not keep her puppies longer to give birth to larger and more mature ones although she has sufficient room for it (Figure 2). The precocial very large and “fully” functional (Martin, ceboids Martin,

neonate ofother

non-primates

at birth and is delivered

1983). Among anthropoid

primates,

(the cow for instance)

is

through a rather narrow pelvis

the neonate of cercopithecids

and especially

often has a large head to be delivered through a narrow pelvis (Schultz, 1969; 1983) (Figure 3). The great apes have a larger pelvis and yet deliver a rather small

fetus through

it (Schultz,

1969).

Figure 2. X-ray view of pregnant through the wide pelvis.

bitch at term. Two fetal heads,

indicated

by arrows.

will easily go

While the pelvic shape of all these mammalian species does not adapt to the gestational duration or to fetal size, it adapts very well in different animals to any attempt for sitting posture, erect posture, and even bipedal locomotion. done by Reynolds (193 1).

This is well demonstrated

in the study

OBSTETRICS

AND

THE PELVIS

Figure 3. X-ray view of abdomen of pregnant Rhesus monkeys at term taken during aortic angiography. In monkey on left, the fetal head could not go through the pelvis and a cesarean section was necessary. In monkey on right, the pelvis easily accommodated the fetal head and vaginal delivery was possible. The fetal heads are indicated with arrows.

Much

less is known about

the obstetrical

pelvis of the australopithecines.

Only two

pelves, sufftciently Australopithecus

complete to warrant obstetrical discussion, exist today: the pelvis of afarensis (AL 288-l) which was described by Johanson et al. (1978) and

Johanson

& White

described

by Howell (1978)

(1979),

and the one of Australopithecus and Robinson

(1972).

africanus

The relationship

pelvis and the fetal head of “Lucy” was studied by Berge et al. (1984). & Lovejoy

(1986)

investigated

not only AL 288-1,

(STS

14) which was

between the maternal More recently Tague

but also extended

their comparative

study from the great apes to Homo sapiens. These last two studies describe the platypelloidy of “Lucy”‘s pelvimetry

pelvis and argue in favor of a difhcult delivery. Leutenegger (1972) studied the of STS 14 and presented a different view by concluding that, in the

australopithecines, delivery was quick and easy. It is in the modern human species that the mechanics of obstetrical delivery have been the most extensively studied (Oxorn, 1980; Pritchard & MacDonald, 1980). Cephalic presentation is the rule in all primates (Hartman, 1928; Elder & Yerkes, 1936; Yerkes & Elder,

1937). While in pongids the mechanism

of birth is performed

in the sagittal axis of

252

M. M. ABITBOL

the pelvis and of the cranium

(Fox, 1929; Tinklepaugh,

1932; Elder & Yerkes,

1936; Yerkes

& Elder, 1937), it is not so in Homo sapiens where the cranium has to take a different orientation in order to approach the pelvis, because the latter has lost its antero-posterior oblong occur,

shape

(Oxorn,

from sagittal

1980,

Pritchard

to a non sagittal

& MacDonald,

1980).

axis, raises controversy

When

did the transition

among the anthopologists

quoted above. With Homo sapiens, obstetric severity (Caldwell undergone mainly:

& Moloy,

difficulties

are frequent

by the pelvis of modern

Homo sapiens as compared

(1) the inlet is now oblong transversally

by the ventral protrusion thus

reducing

the

antero-posterior

changes

to the pelvis of great apes,

instead of sagitally,

of the sacral promontory;

making the midpelvis very narrow transversally; ventrally,

and they appear to be growing in

1933). These difficulties are related to the anatomical

and this is accentuated

(2) the ischial spines protrude medially, (3) the lower sacrum and coccyx protrude diameter

of the

outlet

(Oxorn,

1980;

Pritchard & MacDonald, 1980). All these changes worked against encephalisation and made obstetrical delivery difficult in modern Homo sapiens. A larger pelvis would certainly be welcome

(Figure

4).

Figure 4. X-ray views of human female pelvis at term. In patient on ieft, fetal head is larger than bispinal diameter and a cesarean section was necessary. In patient on right, fetal head could easily go through pelvis and vaginal delivery was possible. Fetal heads indicated with arrows.

It does not appear that the shape of the pelvis during the evolution of the species has been oriented primarily toward obstetric purposes. The shape of the pelvis during hominoid evolution and most specifically during hominid evolution was more geared toward adaptation to the progressive use of erect posture, of biped locomotion, and of support ofviscera, than to solving obstetric problems. The present pelvis had more or less taken its permanent shape 3 to 4 million years ago, before the beginning of encephalization (Schultz, 1969; Leutenegger, 1972) at the time when there were no obstetric difficulties.

OBSTETRICS

Obstetric

shaping

AND

THE

253

PELVIS

of the pelvis, if any, started much later when encephalization

was well

advanced,

i.e. with the most archaic Homo saeiens, ca. 0.35 million years ago. Erect posture,

therefore,

was the first factor to impose on the pelvis, because it came primarily. Obstetric on the pelvis came later . maybe too late to influence greatly the pelvis,

requirements

almost entirely shaped already for erect posture. The literature often mentions the conflict between the requirements

of erect posture and

the fetal head. If there is such a conflict from the evolutionary point of view, it is “won” by erect posture, which had a major impact on the pelvis, in view of the aforementioned points. Reynolds (1931) d ocumented how the pelvis is very responsive to any attempt at erect posture in a variety of animal species. Stern & Susman (1983) have demonstrated that although erect posture and bipedal locomotion ofthe early australopithecines were not “perfect”, they were sufficient to give to the pelvis its essential hominid features of low, broad ilium, etc. During this study the implication role and

that

implication

obstetrics

played

was not that erect posture played a solo

no role in the present

rather is that erect posture played a primary

one. Lovejoy et al. (1973) has shown that the differences female pelves are similar to the differences To

review

obstetrical

those

pelvic

requirements

features

that

formation

of the pelvis.

role and obstetrics

between modern human male and and Homo sapiens.

between australopithecines have

and, in the judgement

been

The

a secondary

attributed

by different

of the author,

authors

to

really were due to erect

posture: (1) Widening

of the pelvis with an increased

general widening

leverage to the muscles (2) Widening diameter

transverse

diameter

of the inlet was due to

of the whole trunk as a result of erect posture and to give a better around the hip joint

of the sacrum

contributed

(Reynolds,

further

1931; Schultz,

to the widening

of the pelvis; the sacrum enlarged in all dimensions

needed a wide and firm base of support

(Schultz,

of the transverse

because the erect trunk

1969).

(3) The sacrum and, in fact, the whole pelvis, retained its original horizontal face of the erect posture of the rest of the hominid body (Robinson, the sacrum did not move backward

species, joints joint.

became

position, in

1972). Therefore

to make more room for the encaphalizing

the fetus: it stayed horizontal, and the upper trunk turned vertically lumbosacral joint (Robinson, 1972). (4) The pubic symphysis

1969).

shorter

in the human

species.

head of

around

the

In the quadrupedal

the weight of the caudad part of the body is carried by both the ilio-sacral

and the symphyseal joint, and is transmitted from all these joints to the hip In the erect posture there is a partial shift of body weight from the pubic

symphysis to the sacroiliac joint. This is the reason why the symphysis important in bipeds than in quadrupeds (Robinson, 1972). (5) The ischiopubic

axis became

thinner

and more elongated:

changes in the whole innominate bone, which are shortening dimension and widening in the lateral dimension (Robinson,

this change

pubis is less is like the

in the crania-caudal 1972).

In summary these five changes, except possibly the last one, are the result of the erect posture and not dictated by obstetric requirements. Encephalization benefited from them while they were occurring because the hominid pelvis could do without such an elongated anteroposterior diameter of the inlet and could use more width in the inlet. Obstetrics more the recipient than the initiator of these pelvic changes.

was

hf.

254 In the human requirements

M. ABITBOL

pelvis, the list of the features

of encephalization

that could be specifically

is composed

male pelvis of Homo sapiens. While

of the differences

the changes

imposed by erect posture,

some of these changes

slightly

further

for obstetrical

general,

the male pelvis is heavier,

possibly

are greater:

the true conjugate

greater in females

females.

significance

(Pritchard longer,

has a greater

male pelvis were solely

& MacDonald,

are about 10 mm

in the female than in the male; in

backward

tilt, the biacetabular

angle is wider; the symphysis

are slight,

In

in shape,

The female pelvic dimensions

and oblique diameters

is broader

1980).

and more triangular

and more cylindrical.

and the transverse

are wider, and the subpubic All these differences

Moloy,

reasons

than in males; the sacrum

the female pelvis, the sacrum diameters

in the human

to the

the female and

in the human female pelvis were carried

more sturdy,

while the female one is lighter, shorter,

attributed

between

very often absent,

because the female pelvis presents a tremendous

and bispinal

pubis is shorter

in

and have only a statistical variety in shapes (Caldwell

&

1933).

Dietary and general living habits during childhood and adolescence also play a role (Table 9). Most likely, early physical work and early pregnancy will have a tendency to distort the primary Moloy,

shape of the female pelvis. As shown by previous authors

1933; Trinkaus,

1984), the human pelvis is sensitive and responsive

of erect posture and bipedal likely, therefore,

locomotion,

and in fact responds

early physical work and early pregnancy

pelvis and may contribute

(Caldwell

&

to the demands

to any strong impact.

Most

have a strong effect on the female

to its final shape and to the differences

between male and female

pelves.

Summary Based

on study

individual

of 500

the same observation dimensions

patients

during

or total pelvic dimensions

weight. Positional

behaviour

occurred

there

is no relationship

of gestation

can be made in all mammals,

appear to play a role in the duration

of birth is completely

early in hominid

evolution.

Obstetrical

between

or birth weight. In general,

where other factors

of gestation

besides

and the determination

played a major role in determining

Homo sapiens where the mechanism transition

pregnancy,

and duration

pelvic of birth

the pelvic shape of modern

different

from that in apes: the

requirements

played a minor

role.

Acknowledgements I am grateful to Dr Jack

Stern, Department

of Anatomical

of New York, Stony Brook, for reading this manuscript comments

and suggestions

Sciences

at the State University

and offering helpful criticisms.

of the editor and of the reviewers have been markedly

The

useful in

the present version of this manuscript.

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