Theories of Onset of Labour

THEORIES OF ONSET OF LABOUR. NORMAL & ABNORMAL UTERINE ACTION, PROLONGED LABOUR.

INTRODUCTION

During pregnancy, the uterus is a quiescent capacitance organ

During labour, it becomes an efficient contractile unit, with the aim of effecting vaginal delivery

There are several theories explaining the trigger for this change

DEFINITION OF LABOUR Sequence of uterine contractions that results

in effacement of the cervix and voluntary bearing down efforts leading to the expulsion per vagina of the product of conceptus

Highlights: the onset of painful, regular contractions more than one every ten minutes with progressive cervical effacement and

dilatation accompanied by descent of the presenting part May or may not be associated with the

passage of a show or rupture of membranes

Show: blood-stained cervical mucous plug, expelled from the vagina. Often heralds onset of labour

Rupture of membranes:Breakage of the chorio-amniotic membrane with release of amniotic fluid (drainage of liquor).

EVENTS PRECEDING LABOUR Lightening: settling of fetal head into

pelvic brim, usually 2 or more weeks before labour especially in primigravida

Prelabour: last 4 wks of pregnancy; consisting of:

Increasing uterine contractilityBraxton-Hicks contractions: irregular

painless uterine contractions with slowly increasing frequency, in last 4-8 weeks of pregnancy, with lack of cervical changes

Cervical softening (ripening)

THEORIES OF ONSET

General theoremsWithdrawal of pregnancy

maintenanceUterotonic induction of labourA signal from the mature fetus to

initiate labour

WITHDRAWAL OF PREGNANCY MAINTENANCE

Progesterone (the pro-gestation hormone) level is high during pregnancy, its ratio to oestrogen ↓ses at term (P:E ratio)

Theory is supported by studies in mammals

In primates, however, progesterone is not withdrawn till after delivery of placenta

UTEROTONIC INDUCTION OF LABOUR Oxytocin is the major uterotonic hormone

(such that drugs with similar function are called “oxytocics”)

Its level is fairly constant during pregnancy, but ↑ses throughout labour (incl. preterm)

Oxytocin receptors in myometrium ↑ 6-fold by 13-17wks, 80-100 fold by end of pregnancy

P ↓ formation of receptors, E ↑ses it Greatest distribution is in fundus of uterus

UTEROTONIC INDUCTION OF LABOUR

Prostaglandins: Are only one group of compounds involved in labour, yet have an essential command role

Without PGs, labour is impossible; labour is irresistible when they’re abundant

PGF2α is the primary PG generating myometrial contractility

PGE2 is important to cervical ripening Main sources are decidua & amnion,

respectively Concentration ↑ses in late pregnancy

A SIGNAL FROM THE MATURE FETUS TO INITIATE LABOUR

Sheep model of parturition: Human placenta converts cortisol to

oestrogen The fetal adrenals produce these

steroids, transported to placenta through fetal blood

Classical studies (1968) in the sheep, showed that adrenalectomy or hypophysectomy of the fetus (but not the ewe) caused delay in initiation of parturition

FOETAL SIGNAL Contrarily, infusion of corticotropin or

cortisol into sheep fetus earlier in pregnancy caused preterm delivery

The deduction was made that the sheep fetus provides a signal through the hypothalamo-pituitary-adreno-placental axis

The key component appears to be the fetal brain; the signal, presumably CRH

CRH→ACTH →cortisol Foetal anencephaly causes prolonged

labour

FOETAL SIGNAL Cortisol produces maturation of fetal

lungs, which along with the kidneys, modify contents of amniotic fluid

This activates foetal membranes with respect to prostaglandin synthesis

All this give rise to an ‘endocrine crosstalk’ between foetus, placenta, membranes & uterus

“PLAYERS” IN LABOUR

PassagePassengerPowers

THE PASSAGE

Consists of all the structures forming the birth channel

Soft tissue: muscles, ligaments, fat, fascia

Bony pelvis: typesGynaecoid (has widest diameters)Android (male type)Andropoid (persistent OP position)Platypelloid (flat, likely to cause

obstructed labor)

COMPONENTS OF THE BONY PELVIS

Pelvic brim (pelvic inlet)

Mid-cavity Pelvic outletAssessment of its

capacity are done by pelvimetry Clinical Radiological

(X-ray/CT)

THE PASSENGER

Fetus Placenta & membranesThe foetal lie, presentation, position, attitude &

size all contribute to the progress of labourThe well-flexed foetal head presents a widest

diameter of 9.5 cm, which is the head’s smallest possible diameter

POWERS

• Primary forces: uterine myometrial contractions

• Secondary forces: Contraction of voluntary muscles of diaphragm and abdominal walls

UTERINE CONTRACTIONS

Electrical pacemaker is in the cornual region Contractions are measured by their

frequency, duration & strength Adequate contractions are

3-4 in 10 minutes Lasting 45-60 seconds each Strong

Clinically, indentation can’t be made over uterus Amplitude of 60 mmHg with electronic monitoring

MECHANISMS OF LABOUR

The various positional changes that foetal head makes as it negotiates the birth canal

Descent (occurs through all the mechanisms) Engagement Flexion Internal rotation Extension Restitution External rotation

ENGAGEMENT Station refer to the

position of an arbitrary point on the presenting part to the ischial spines

Ranges from -3 to +2; each rep. a distance of 1cm from the spines (station 0)

Engagement is when widest diameter of head enters pelvis (i.e. vertex is at station 0)

DESCENT, FLEXION, INTERNAL ROTATION & EXTENSION

RESTITUTION & EXTERNAL ROTATION

NORMAL LABOUR

Adequate contractions Does not last more than 12 hours Oxytocics are not required Ends in a spontaneous vaginal delivery Singleton foetus Cephalic presentation Obviously, a retrospective diagnosis

STAGES OF LABOUR

First stage: from undilated cervix till full dilatation; i.e. 0cm to 10 cm Latent phase: from 0 to 3 cm, & full cervical

effacement Active phase: from 3 cm to 10 cm

Second stage: from full dilatation to expulsion of the fetus Propulsive Expulsive

Third stage: from delivery of foetus to delivery of placenta

1ST STAGE

Latent phase: Lasts up to 14 hrs in multiparous

women, 20 hrs in nulliparous Active phase:

Cervix dilates abt 1 cm/hour (faster in mutipara)

Presenting part descends at abt 1 cm/hour (measured as descent par abdomen or engagement par vaginum—the former is more accurate)

ABNORMALITIES OF 1ST STAGE Prolonged latent phase

Therapeutic rest Oxytocin

Dysfunctional contractions (hypotonic & hypertonic inertia)—oxytocin

Protraction of dilatation (<1cm/hr) ARM, oxytocin

Protraction of descent (<1cm/hr)—oxytocin Arrest of dilatation (> 2 hrs)

ARM, oxytocin, C/S Arrest of descent (>2hrs)

Operative vaginal delivery, C/S

ABNORMALITIES OF 2ND STAGE

Prolonged 2nd stage: >1 hr in multips, >2 hrs in nullips (add 1 hr, if on epidural anaesthesia) Normally lasts abt 30 mins

Deep transverse arrest (ass. with android type pelvis); if head cannot internally rotate due to narrow mid-cavity

Secondary uterine inertia due to maternal exhaustion (dehydration, ketosis)Resuscitate, oxytocin Persistent OP positionmay need assisted delivery or C/S

THIRD STAGE

Most dangerous complication is haemorrhage Prevented by active management of the 3rd

stage of labour Early clamping of cord Administration of oxytocic at delivery of anterior

shoulder (oxytocin, syntometrine) Delivery of placenta by controlled cord traction

3rd stage lasts abt. 5 mins; is prolonged if it lasts more than 30 minutes May require manual removal of placenta under

anaesthesia

CONTROLLED CORD TRACTION

PROLONGED LABOUR: CAUSES

Wrong diagnosis of labour Excessive sedation Cervical dystocia Full bladder Macrosomic infants Malposition especially persistent

occipitoposterior Malpresentation (breech, face, brow) Inadequate pelvis (fetopelvic disproportion) Congenital anomalies eg anecephaly Pelvic tumours e.g. uterine fibroids

PROLONGED LABOUR: INTERVENTION

Identify cause & treat appropriately

RehydrationOxygen supplementationLeft lateral maternal positionAugmentation of labour

PROLONGED LABOUR: PREVENTION

Strict criteria for definition of labour

Use of partograph for women in labour

Active management of labourOne to one midwifery care

THE EXPECTED END OF LABOUR...

....and the end of the lecture!