Obstetrics Simplified - Diaa M. EI-Mowafi

Normal Labour

Definitions

Labour is the process by which a viable foetus i.e. at the end of 28 weeks or more is expelled or is going to be expelled from the uterus.

Delivery means actual birth of the foetus.

The following criteria should be present to call it normal labour:

• Spontaneous expulsion,
• of a single,
• mature foetus,   
• presented by vertex,    
• through the birth canal,  
• within a reasonable time (not less than 3 hours or more than 18 hours), 
• without complications to the mother,
• or the foetus.

Cause of Onset of Labour

It is unknown but the following theories were postulated:

Hormonal factors

• Oestrogen theory:
  • During pregnancy, most of the oestrogens are present in a binding form. During the last trimester, more free oestrogen appears increasing the excitability of the myometrium and prostaglandins synthesis.
• Progesterone withdrawal theory:
  • Before labour, there is a drop in progesterone synthesis leading to predominance of the excitatory action of oestrogens.
• Prostaglandins theory:
  • Prostaglandins E2 and F2α are powerful stimulators of uterine muscle activity. PGF2α was found to be increased in maternal and foetal blood as well as the amniotic fluid late in pregnancy and during labour.
• Oxytocin theory:
  • Although oxytocin is a powerful stimulator of uterine contraction, its natural role in onset of labour is doubtful. The secretion of oxytocinase enzyme from the placenta is decreased near term due to placental ischaemia leading to predominance of oxytocin’s action.
• Foetal cortisol theory:
  • Increased cortisol production from the foetal adrenal gland before labour may influence its onset by increasing oestrogen production from the placenta.

Mechanical factors

• Uterine distension theory:
  • Like any hollow organ in the body, when the uterus in distended to a certain limit, it starts to contract to evacuate its contents. This explains the preterm labour in case of multiple pregnancy and polyhydramnios.
• Stretch of the lower uterine segment:
  • by the presenting part near term.

CLINICAL PICTURE OF LABOUR

Prodromal (pre-labour) stage

The following clinical manifestations may occur in the last weeks of pregnancy.

• Shelfing:
  • It is falling forwards of the uterine fundus making the upper abdomen looks like a shelf during standing position. This is due to engagement of the head which brings the foetus perpendicular to the pelvic inlet in the direction of pelvic axis.
• Lightening:
  • It is the relief of upper abdominal pressure symptoms as dyspnoea, dyspepsia and palpitation due to:
    • descent in the fundal level after engagement of the head and
    • shelfing of the uterus.
• Pelvic pressure symptoms:
  • With engagement of the presenting part the following symptoms may occur:
    • Frequency of micturition,
    • rectal tenesmus and   
    • difficulty in walking.
• Increased vaginal discharge.
• False labour pain:

These are differentiated from true labour pain as follow:

Onset of Labour

It is characterised by:

• True labour pain.
• The show:
  • It is an expelled cervical mucus plug tinged with blood from ruptured small vessels as a result of separation of the membranes from the lower uterine segment. Labour is usually starts several hours to few days after show.
• Dilatation of the cervix:
  • A closed cervix is a reliable sign that labour has not begun. In multigravidae the cervix may admit the tip of the finger before onset of labour.
• Formation of the bag of fore-waters:
  • It bulges through the cervix and becomes tense during uterine contractions.

STAGES OF LABOUR

Labour is divided into four stages:

• First stage

  • It is the stage of cervical dilatation.

  • Starts with the onset of true labour pain and ends with full dilatation of the cervix i.e. 10 cm in diameter.

  • It takes about 10-14 hours in primigravida and about 6-8 hours in multipara.

• Second stage

  • It is the stage of expulsion of the foetus.

  • Begins with full cervical dilatation and ends with the delivery of the foetus.

  • Its duration is about 1 hour in primigravida and ½ hour in multipara.

• Third stage

  • It is the stage of expulsion of the placenta and membranes.

  • Begins after delivery of the foetus and ends with expulsion of the placenta and membranes.

  • Its duration is about 10-20 minutes in both primi and multipara.

• Fourth stage

  • It is the stage of early recovery.

  • Begins immediately after expulsion of the placenta and membranes and lasts for one hour.

  • During which careful observation for the patient, particularly for signs of postpartum haemorrhage is essential. Routine uterine massage is usually done every 15 minutes during this period.

First Stage

Causes of cervical dilatation

• Contraction and retraction of uterine musculature.
• Mechanical pressure by the forebag of waters, if membranes still intact, or the presenting part, if they had ruptured. This in turn will release more prostaglandins which stimulate uterine contractions and cervical effacement.
• Softness of the cervix which has occurred during pregnancy facilitates dilatation and effacement of the cervix.

Mechanism of cervical dilatation

• In primigravidas, the cervical canal dilates from above downwards i.e. from the internal os downwards to the external os. So its length shorts gradually from more than 2 cm to a thin rim of few millimetres continuous with the lower uterine segment. This process is called effacement and expressed in percentage so when we say effacement is 70% it means that 70% of the cervical canal has been taken up.
• Dilatation of the cervix (external os) starts after complete effacement of the cervix.
• In multigravidas, effacement and dilatation occur simultaneously.
• In normal presentation and position, the head is applied well to the lower uterine segment dividing the amniotic sac by the girdle of contact into a hindwaters above it containing the foetus and a forewaters below it. This reduces the pressure in the forewaters preventing early rupture of membranes. After full dilatation of the cervix the hind and forewaters become one sac with increased pressure in the bag of forewaters leading to its rupture.

Phases of cervical dilatation

• Latent phase:
  • This is the first 3 cm of cervical dilatation which is slow takes about 8 hours in nulliparae and 4 hours in multiparae.
• Active phase:
  • It has 3 components:
    • acceleration phase,
    • maximum slope, and
    • deceleration phase.

The phase of maximum slope is the most detectable and the two other phases are of shorter duration and can be detected only by frequent vaginal examination.

The normal rate of cervical dilatation in active phase is 1.2 cm/ hour in primigravidae and 1.5 cm/hour in multiparae. If the rate is < 1cm / hour it is considered prolonged.

Second Stage

Delivery of the head

• Descent:
  • It is continuous throughout labour particularly during the second stage and caused by:
    • Uterine contractions and retractions.
    • The auxiliary forces which is bearing down brought by contraction of the diaphragm and abdominal muscles.
    • The unfolding of the foetus i.e. straightening of its body due to contractions of the circular muscles of the uterus.
• Engagement:
  • The head normally engages in the oblique or transverse diameter of the inlet.
• Increased flexion:
  • As the atlanto-occipital joint is nearer to the occiput than the sinciput, increased flexion of the head occurs when it meets the pelvic floor according to the lever theory.
  • Increased flexion results in:
    • The suboccipito-bregmatic diameter (9.5 cm) passes through the birth canal instead of the suboccipito-frontal diameter (10 cm).
    • The part of the foetal head applied to the maternal passages is like a ball with equal longitudinal and transverse diameters as the suboccipito-bregmatic = biparietal = 9.5 cm. The circumference of this ball is 30 cm.
    • The occiput will meet the pelvic floor.
• Internal rotation:
  • The rule is that the part of foetus meets the pelvic floor first will rotate anteriorly. So that its movement is in the direction of levator ani muscles (the main muscle of the pelvic floor) i.e. downwards, forwards and inwards.
  • In normal labour, the occiput which meets the pelvic floor first rotates anteriorly 1/8 circle.
• Extension:
  • The suboccipital region lies under the symphysis then by head extension the vertex, forehead and face come out successively.
  • The head is acted upon by 2 forces:
    • the uterine contractions acting downwards and forwards.
    • the pelvic floor resistance acting upwards and forwards so the net result is forward direction i.e. extension of the head.
• Restitution:
  • After delivery, the head rotates 1/8 of a circle in the opposite direction of internal rotation to undo the twist produced by it.
• External rotation:
  • The shoulders enter the pelvis in the opposite oblique diameter to that previously passed by the head. When the anterior shoulder meets the pelvic floor it rotates anteriorly 1/8 of a circle. This movement is transmitted to the head so it rotates 1/8 of a circle in the same direction of restitution.

Delivery of the shoulder and body

The anterior shoulder hinges below the symphysis pubis and with continuous descent the posterior shoulder is delivered first by lateral flexion of the spines followed by anterior shoulder then the body.

Third Stage

After delivery of the foetus, the uterus continues to contract and retract. As the placenta is inelastic, it starts to separate through the spongiosa layer by one of the following mechanisms:

Schultze’s mechanism (80%)

• The central area of the placenta separates first and placenta is delivered like an inverted umbrella so the foetal surface appears first followed by the membranes containing small retroplacental clot.
• There is less blood loss and less liability for retention of fragments.

Duncan’s mechanism (20%)

• The lower edge of the placenta separates first and placenta is delivered side ways.
• There is more liability of bleeding and retained fragments.

The 3rd stage is composed of 3 phases:

• Placental separation.
• Placental descent.
• Placental expulsion.

PHYSIOLOGICAL EFFECTS OF LABOUR

On the Mother

• First stage:
  • minimal effects.
• Second stage:
  • Temperature: slight rise to 37.5oC.
  • Pulse: increases up to 100/min.
  • Blood pressure: systolic blood pressure may rise slightly due to pain, anxiety and stress.
  • Oedema and congestion of the conjuctiva.
  • Minor injuries: to the birth canal and perineum may occur particularly in primigravidas.
• Third stage:
  • Blood loss from the placental site is 100-200 ml and from laceration or episiotomy is 100 ml so the total average blood loss in normal labour is 250 ml.

On the Foetus

Moulding

The physiological gradual overlapping of the vault bones as the skull is compressed during its passage in the birth canal.

One parietal bone overlaps the other and both overlap the occipital and frontal bones so fontanelles are no more detectable. It is of a good value in reducing the skull diameters but severe and / or rapid moulding is dangerous as it may cause intracranial haemorrhage.

Caput succedaneum

• It is a soft swelling of the most dependent part of the foetal head occurs in prolonged labour before full cervical dilatation and after rupture of the membranes.
• It is due to obstruction of the venous return from the lower part of the scalp by the cervical ring.
• Large caput may:
  • obscure the sutures and fontanelles making identification of the position  difficult. This can be overcomed by palpation of the ear,
  • give an impression that the head is lower than its true level.
• Artificial caput succedaneum (chignon): is induced during vacuum extraction.
• Caput succedaneum disappears spontaneously within hours to days of birth.
• As it is a vital manifestation, so it is not detected in intrauterine foetal death.

The presence of caput indicates that:

• the foetus was living during labour,
• labour was prolonged and difficult,
• the attitude of foetal head during labour can be expected as caput is present in the most dependant part of it.

Links

• Labor, delivery : Guidelines, reviews

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Edited by Aldo Campana,