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NORMAL LABOUR

Introduction, maternal and fetal anatomy

By reference textbook

Suhaila Al -Shaikh Obstetrics by ten teachers
20 th ed (2017 )

Learning objectives

1. The student should know the types of female
pelvis.
2. understand the importance of the dimensions of
the bony pelvis of the pregnant woman in
determining the progress of labour and the mode
of delivery.
3. How to assess pelvic dimensions.
4. Know the dimensions of the fetal skull.
5. Understand how the attitude of the fetal head
affects these dimensions.

labour

Labour or human parturition is the
physiological process that results in
birth of a baby , delivery of the placenta
and the signal for lactation to begin.
POWER
PASSAGES
PASSENGER
The 3 Ps ??

Labour

Health professionals who manage labour should understand that:

The first important step is to recognize when labour has started .

Labour is then divided into three stages:
the first stage begins with diagnosis of the onset of labour and is
complete when full cervical dilatation ?? (how many cm ) has been
reached;
the second stage begins with full cervical dilatation and ends with
birth of the baby;
and the third stage begins with birth of the baby and ends with
complete delivery of the placenta and membranes.
Complications can occur during any of the three stages and can
be divided into maternal and fetal -neonatal complications.

• Labour can be defined as the

process by which regular painful
contractions bring about
effacement and dilatation of the
cervix and descent of the
presenting part, leading to
expulsion of the fetus and the
placenta from the mother.

A doctor or midwife who manages

labour must be aware of the normal
anatomy and physiology of the
mother and fetus, what
distinguishes an abnormal from a
normal labour , and when it is
appropriate to intervene

Anatomy

Maternal bony pelvis
and fetal head

Bony pelvis

• The bony pelvis is made of 4
bones: the sacrum, coccyx, and
2 innominate bones which are
(composed of the ilium,
ischium, and pubis). These are
held together by the SIJ, SP, and
the SCJ joints.

The bony

pelvis.

(a) Inlet: Bean

shaped.
(b) Mid -cavity:
Circular.
(c)Outlet:Diamond
shaped .

The bony

pelvis.

(a) Inlet: Bean

shaped.
(b) Mid -cavity:
Circular.
(c)Outlet:Diamond
shaped .

The bony

pelvis.

(a) Inlet: Bean

shaped.
(b) Mid -cavity:
Circular.
(c)Outlet:Diamond
shaped .

The
pelvic axis
describes
imaginary
curved line,
a path that
the centre
of the fetal
head must
take during
its passage
through the
pelvis

The pelvic brim or inlet

The pelvic mid - cavity
The pelvic mid -cavity can be described as an area
bounded in front by the middle of the symphysis
pubis,
on each side by the pubic bone, the obturator
fascia and the inner aspect of the ischial bone
and spines,
and posteriorly by the junction of the second and
third sections of the sacrum.

The pelvic cavity is almost rounded

T - diam. = 12 cm
A - P diam. = 12 cm.

Ischial spines (in the midcavity )

View from below
View from above

The ischial spines are palpated vaginally

and are used as landmarks to:
1. assess the descent of the head on vaginal
examination (station of the presenting
part).
2. providing an anaesthetic block to the
pudendal nerve which is needed for
instrumental delivery.

The pelvic outlet

The pelvic outlet is bounded
in front by the lower margin of the
symphysis pubis,
on each side by the descending ramus of
the pubic bone, the ischial tuberosity and
the sacrotuberous ligament,
and posteriorly by the last piece of the
sacrum.
The AP diameter of the pelvic outlet is
13.5 cm and the transverse diameter is 11
cm

• Pelvic shape or type

• Maternal stature & ethnicity
• P revious pelvic fractures and metabolic bone disease ,
such as rickets
• And as the loosening of pelvic ligaments towards the
end of the third trimester by relaxin , the pelvis
becomes more flexible and these diameters may
increase during labour .
• Some favourable maternal positions in labour (e.g.
squatting or kneeling).
Factors affecting pelvic dimentions

1. the obstetric conjugate of the pelvic

inlet ( A - P dimension) : 11 cm
2. the bispinous diameter (cavity width):
10.5 cm in the midcavity .
3. the bituberous diameter 11 cm (the
pelvic outlet width)
4. the the sacral concavity and its length
5 . the subpubic angle (arch)
Pelvic diameters: These represent the space
available for the fetal head when it passes
through the pelvis during labour

Pelvic shapes (types)

We have 4 types or shapes of the
bony pelvis and these are:
gynecoid
android
anthropoid
platypelloid .

And their associated obstetric

outcomes

Gynecoid pelvis

Is the most common 50 % associated with high success

Android pelvis

Comprise to 30 % & it predispose to failure of
rotation and deep transverse arrest

anthropoid pelvis

It predispose to an occipito -posterior (OP)
position.

Platypelloid pelvis

is associated with an increased risk of obstructed labour
due to failure of the head to engage, rotate or descend .

The perineum

The final obstacle against the desent of
the fetus during labour is the
perineum. It may be involved in a
second - degree perineal tear and an
episiotomy in primiparous women .
While in multiparous it may remain
intact during vaginal delivery

Dimensions of the fetal skull

• The fetal head is the largest and
the least compressible part of the
fetus
• The fetal skull consists of a base
and a vault (cranium) which
consists of the occipital, parietal,
frontal and temporal bones

The fetal skull from the superior

view

The fetal skull from the lateral

view

these are easily compressible

and interconnected by
membranes and these features
allow molding to occur which
means the overlap of these
bones under pressure and
changing their shape to
conform to maternal pelvis
during vaginal delivery

moulding of the fetal skull

The effect of fetal attitude on the presenting diameter


رفعت المحاضرة من قبل: Hatem Saleh
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