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Obstetrical anatomy

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General

The height of a pregnant woman is very vital in the

elucidation of the outcome of labor. Generally,

a woman below the height of 5ft is considered to

have a small pelvis and will have difficulty in

the delivery of baby.

In the condition recognised as dystocia dystrophia syndrome,

the patient is short, very sturky and has an android (male)

type of pelvis. The patient is very likely to have difficulty in delivery.

 

Fundal height

Fundal height measurement is utilised in the determination

of the age of gestation by the method of palpation in routine

obstetrical care. The fundus rises from the false pelvis and

is palpable through the anterior wall of the abdomen at the

hypogastrium at 8-12 weeks of gestation.

 

At the week 16 weeks of gestation the uterine fundus has

reached midway between the umbilicus and the pubic symphysis.

At 20 weeks it has reached the umbilicus. Thereafter it increases

by about 2.5cm for every 2 weeks. It reaches midway between

the umbilicus and the xiphoid process at 28 weeks of gestation.

Pelvimetry

There are three anatomical cavities of the lesser pelvis

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Inlet

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Midcavity

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Outlet

 

Diagonal conjugate diameter

 

In clinical assessment of the inlet dimensions, the

diagonal conjugate diameter is the single most important

measurement to be taken. It measures the diameter

from the inferior surface of the pubic symphysis to

the sacral promontory. This is best assessed by

vaginal (pelvic) examination with the use of the

index finger. The examine first of all measures his own index

finger length and determines during examination whether

he can feel for the sacral promontory or not.

  1. If the examining finger just touches the sacral promontory
    then the diagonal conjugate diameter is equal to the
    length of he examining finger.

  2. If the examining finger does not reach the sacral promontory
    then the diagonal conjugate diameter is less then the length
    of the examining finger.

 

Midcavity

The most used methods of clinical assessment of the dimensions

of the midcavity are

1.   Morphology of ischial spines. The spines may be heavy or small.

If h If heavy then they should decrease the transverse diameter

ter of the midcavity and probably cause transverse arrest of

      the head of the baby during labour.

2.      Sacrospinous diameter is measured by insinuating two fingers

     in the interval between the ischial spine and the lateral border

of  of the sacrum during pelvic examination. The width of the two

     examining fingers are initially noted to determine the approximate

     diameter of the   sacrospinous interval.

 

     Outlet

 

     Intertubersichial diameter which measures the diameter between the

     ischial tuberosities. It is best assessed with the knuckle of the examining

   hand pressed firmly in the interval between the ischial tuberosities.

   It should be about 8cm or above.

2.      Subpubic angle is assessed by the use of the angle between

     the index finger and the thumb in determining the angle between the

      two inferior rami of the pubis

 

The following problems may be identified by the use of clinical

assessment in labor

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1.      General contraction of all 3 types of cavities-
possible justominor gynecoid pelvis

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2.      Contraction only at the outlet which may point to
an android pelvis

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3.      Contraction at the midcavity with heavy spines which
may be indicative of anthropoid pelvis.

 

Android pelvis is associated with outlet dystocias (difficulty
labour caused by outlet problem) and engagement of the fetal
head may be delayed by encroachment of  the sacral
promontory. Occipitolateral and occipitoposterior positions a
re common (see below).

Gynecoid pelvis is the normal female pelvis but it may be

generally contracted in the gynaecoid pelvis of a small woman

which is called the justominor variety.

Anthropoid pelvis is associated with transverse arrest of the head

of the fetus in labour because of the usual prominence of the
ischial spines.

Flat pelvis is always associated with severe with severe labour
difficulty and its dimensions of the inlet are heavily contracted.

Anatomy of labour

During labour a curve is formed by the birth canal called the

curve of Carus. This curve is formed by the prolongation of 

perineum below its normal position inferiorly and by the vagina

superiorly.

 

The shape of the pelvic floor forms a gutter which is directed

downwards and anteriorly. It is the bend of this gutter that is

accentuated with the formation of the Carus curve.

 

Once the head of the baby hits the pelvic floor the pelvic

gutter directs the head along its own configuration,

hence the rotation of head.

 

As the head hits the pelvic floor, internal orientation occurs and the head

rotates through an angle of 45o. This rotation brings the head into a

direct occipitoanterior position, a position it keeps till it crowns.

Further descent will cause the head to be delivered through a

movement of extension so that the occiput is delivered first.

 

As the shoulder enters the pelvic floor and proceeds to pass

through the same process as the head,

At the stage in which the shoulder is at oblique position

(i.e. left acromioanterior position) the head is already delivered

and it proceeds to undergo restitution by which it rotates through

an angle of 45o to the left in order to be in alignment with the orientation

of the shoulder at the pelvis.
 

The head then undergoes external

rotation to an occipitolateral position so that the occiput lies next

to the left thigh.

The anterior shoulder then slips under the symphysis pubis and is

delivered. This is followed by the posterior shoulder. The rest of

the baby’s body allows the delivery of the shoulders. This normal

process of labour may develop complications at any stage during

labour.
 

Head crowns at perineum
Heads performs restitution  
Anterior shoulder delivered
Posterior shoulder delivered
Torso delivered
Cut umbilicus
Remove placenta
Remove placenta

Episiotomy is a procedure used to widen the vagina in

order to have an easy vaginal delivery. It is recommended in

all first deliveries (primigravids) and all cases of soft tissue

dystocias (difficult labor caused by soft tissue rigidity). A

Mediolateral, B J shaped mediolateral C Median D Schuchardt.

Episitomy incision passes through mucous membrane of vagina,

superficial transverse perineal muscle and perianal fat. The

median incision passes through the central perineal tendon. A lateral

incision is avoided because it destroys the greater vestibular gland.
 

Delivery is performed after episiotomy
A repair is done after delivery

 

The above represents the normal presentation in parturition.

Abnormal presentation may include breech delivery

in which the sacrum rather than the head is the presenting part.

 

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