Labour is the process of birth. In response to uterine contractions the lower segment stretches and thins, the cervix dilates, the birth canal is formed and the baby descends through the pelvis.
UTERINE ACTION
Cervical dilatation
‘Show’ and formation of forewaters
LABOUR — THE BIRTH CANAL
First Stage…
start to full dilatation of the cervix.
Second Stage…
full dilatation to birth of baby.
Third Stage…
birth of baby to delivery of placenta (afterbirth).
AT THE BEGINNING OF LABOUR
BIRTH CANAL AT BEGINNING OF SECOND STAGE
THE MECHANISM OF LABOUR
NORMAL MECHANISM
DIAGNOSIS OF LABOUR
True labour
False or Spurious Labour
LABOUR IS RECOGNISED BY:
PROGRESS IN LABOUR
PROGRESS IN LABOUR IS GAUGED BY:
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Normal Labour
The fibres of the myometrium Contract and Relax like all muscle.
In labour when the muscle fibres relax they do not return to their former length but become progressively shorter: this is Retraction.
The uterine capacity is thus progressively reduced and the thickness of the uterine wall increased.
As labour progresses the contractions increase in frequency, strength and duration. The muscle of the lower segment of the uterus (see Chapter 2) is thin and relatively passive, and the cervix consists mainly of fibrous connective tissue.
Effacement is most striking in the primigravida. In the parous patient dilatation and effacement usually occur together.
The effacement and dilatation of the cervix loosens the membranes from the region of the internal os with slight bleeding and sets free the mucus plug or operculum. This constitutes the ‘show’ and allows the formation of forewaters, the amniotic sac pushing against the cervix.
Labour is divided into Three Stages:
The fetus is descending during first and second stages of labour.
The birth canal is formed by dilatation of the cervix and vagina and by stretching and displacement of the muscles of the pelvic floor and perineum.
The bladder is pulled above the pubis because of its attachment to the uterus; the urethra is stretched and the bowel is compressed.
Stretching and displacement viewed from below (after delivery)
Illustrated is the mechanism of labour where the vertex presents in the left occipito-lateral (LOL) position.
Engagement is the descent of the presenting diameters through the pelvic brim.
The leading part — the vertex — is now near the level of the ischial spines.
Descent continues and the occiput rotates in the cavity of the pelvis anteriorly to the right oblique diameter, bringing the occiput to the left obturator foramen anteriorly.
Now in left occipito-anterior (LOA) position.
The LOA position is partly attributed to the presence of the sigmoid colon in the left posterior quadrant of the pelvis.
Note how the neck is twisting.
The occiput rotates to the front. This is Internal rotation. The head is now occipito-anterior (OA). Note twisting of the head and shoulders. The shoulders are in the left oblique of the brim.
It is a maxim that the fetal part which first comes in contact with the pelvic floor rotates anteriorly (Internal rotation).
Rotation is through 45° from oblique and is called Anterior or Short rotation.
The Occiput is now below the symphysis. Further descent of the fetus pushes the head forwards with a movement of extension and the occiput is delivered.
Increasing extension round the pubis delivers the Bregma, Brow and Face.
The head on delivery is oblique to the line of the shoulders. The bisacromial diameter is in left oblique diameter of the cavity.
The bisacromial diameter is the distance between the acromion processes and is 11cm.
The head now rotates to the natural position relative to the shoulders. This movement is known as Restitution.
Descent continues and the shoulders rotate to bring the bisacromial diameter into the antero-posterior diameter of the pelvic outlet.
This descent and rotation causes the head to rotate so that the occiput lies next to the left maternal thigh. This is External rotation.
The anterior shoulder now slips under the pubis and with lateral flexion of the fetal body the posterior shoulder is born. The rest of the body follows easily.
Regular contractions.
‘Show’.
Progressive dilatation and
effacement of cervix.
Irregular contractions.
No ‘Show’.
No progressive dilatation or
effacement of cervix.
1. Palpable uterine contractions which are regular in frequency and intermittent in character. The interval between contractions is 10 minutes or less and each contraction may last half a minute or longer.
The uterus becomes firm and rises, altering the abdominal contour. This is due to the rising forwards of the uterus so that it approximates to the direction of the birth canal. This movement is easier if the patient is upright. Ambulation may therefore give mechanical advantage.
2. ‘Show’. A little blood and mucus discharged from the vagina. This is from separation of the membranes at the lower pole causing bleeding which mixes with the operculum of the cervix.
3. Dilatation of the Cervix. This is accompanied by the formation of forewaters or bag of waters.
Cervical dilatation is gauged by vaginal examination and is expressed in the diameter across the cervix.
2. Dilatation of the cervix