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Labour , physiology of

onset, and stages

By reference textbook

Suhaila Al -Shaikh Obstetrics by ten teachers
20 th ed (2017 ): ch 12 ; p 381 -87

Learning objectives

 Understand the physiology of labour
 Understand factors that influencing uterine
activity
 Physiological changes preparing to parturition
 Define stages of labour and be able to
differentiate normal from abnormal labour
(prolonged or precipitous).

Definition of labour

Labor is defined as the
onset of a sequence of
painful regular uterine
contractions that
results in progressive
effacement and
dilatation of the cervix
with descent of the
presenting part and
voluntary maternal
bearing -down efforts
leading to the
expulsion of the
products of
conception through
the vagina.

Average duration of labour :

• 8 hours in primi
• 5 hours in multiparous

Onset of labour

 The reason or the mechanism behind the
onset of labour is poorly understood
 How the following events happen??
 The cervix firm and closed  soft and
stretchable, open and dilated?
 The cervix long & thick  short and thin
(effaced)?
 The uterus quiescent & relaxed  regularly,
strongly and frequently contracting?

Why labour starts at the end of gestation

why not before or after?
The mother and her baby play a role in this process.
the uterus:
 Myometrial cells ( myocytes ) are interlacing,
synchronized in action
PG
Gap junctions
 Myocytes contract and retract  progressively shorten
 Upper (active and dominant) and lower (passive)
segments

Upper (a ctive) & lower (passive)

segments of the uterus

Uterine contractions of labour

are characterized:
 Involuntary
 Irregular in the beginning then become
regular
 Frequency  2 /10 minutes in early labour
 Frequency  4 -5 /10 minutes in advanced
labour
 Duration  30 – 60 seconds

Cervix: contains myocytes and

fibroblast.
 Collagen
 Fibronectin
 dermatan sulphate (a proteoglycan )
Early in pregnancy: remain closed, firm and
not dilate in response to contractions.
Later (toward term):
Proteolytic activity  soften and ripen and
easily dilate in response to uterine contractions.
 (increased water content) By effect of PG and
other humoral factors

Factors initiating labour

Hormonal factors:
 Progesterone and its receptor
 Estrogen and its receptor
 Progesterone/ Estrogen  ratio
 Cortisol (fetal adrenal)
 CRH (placental)
 Oxytocin (maternal and fetal)
 Fergusson reflex

Mechanical factors:

 Uterine stretching at full term
 Over stretch in multiple pregnancy
and polyhydramniose  premature
labour

physiologic Preparation for Labor

 Lightening : settling of the fetal head into the
brim of the pelvis.

 Braxton Hicks contractions : increase in

frequency.

 Cervical ripening : the cervix begins to

soften and become stretchable.

 Lightening : occurs 2 or more weeks before labor in first

pregnancies.
it does not occur until early labor in multiparous women.
 Clinically, the mother may notice a flattening of the upper
abdomen and increased pressure in the pelvis.

 Braxton Hicks contractions : may occur more

frequently, sometimes every 10 – 20 minutes, and with
greater intensity during the last weeks of pregnancy.
When these contractions occur early in the third
trimester, they must be distinguished from true preterm
labor.
 These are a common cause of false labour .

DIAGNOSIS OF LABOUR

symptoms & signs of labour :
1 - abdominal and back pain
2 - increased vaginal discharge which may be bloody
stained discharge or watery (show).
 (the Show : it means the passage of a small amount of
blood -tinged mucus from the vagina).
3 - there may be nausea and vomiting due to pain
4 - in advanced stage of labour (late 1 st and 2 nd stage) there
is increased pain and urge to (push) bearing down
5 - increased frequency of micturition, and urge for bowel
evacuation when the baby’s head press on the bladder
and rectum
6 - uterine tightening (contractions) by abdominal palpation
on regular intervals 2 -4 /10 minutes
7 - cervical dilatation and effacement by serial vaginal exam

Duration of labour

 There is no ideal length for normal labour
 In nulliparous if > than 12 hrs  prolonged
 In multiparous if > than 8 hrs  prolonged
 Precipitous labour : expulsion of the fetus
within < than 3 hours of the onset of regular
contractions.

STAGES OF LABOUR

Stages of labour :
 First stage: from the time of the
beginning of labour until 10 cm cervical
dilatation .
 Second stage: the period between full
cervical dilatation to delivery of the
baby.
 third stage: the period from the
delivery of the baby to the complete
delivery of the placenta and
membranes.

1
st
stage
 subdivided into two phases, the latent phase
and the active phase.
Latent : from onset of labour  3 -4 cm
duration 3 -8 hours but shorter in
multiparous
Active : from 3 -4 cm up to 10 cm
duration 2 -6 hours being shorter in
multiparous
Rate of cervical dilatation is 1 cm/hour is normal
Being faster in multiparous
If < 1 cm / 2 hours is considered abnormal

Effacement & dilatation in latent phase

Onset of
Labour (latent phase)
Beginning
of active phase

2
nd
stage
2 nd stage is further subdivided into 2 phases:
passive and active 2 nd stage
Passive : the time between full dilatation and
the onset of involuntary expulsive
contractions
duration 1 -2 hrs (for desc . and rotation)
Active 2 nd stage : there is maternal urge to
push and the fetal head is visible at
the perineum
duration 2 hr in primi , 1 hr in multi
Epidural usually cause prolongation of duration

3
rd
stage:
 duration of the third stage is 0 – 30
minutes for all pregnancies.
 Separation of the placenta generally
occurs within 2 – 10 minutes of the end
of the second stage
 If 3 rd stage managed physiologically
without oxytocic interference the
duration is up to 60 minutes

features of normal labour ?

1 . Spontaneous onset at 37 – 42 weeks’ gestation.
2 . Singleton pregnancy.
3 . Cephalic vertex presentation.
4 . No artificial interventions.
5 . Cervical dilatation of at least 1 cm/ 2 hours in the
active phase of first stage .
6 . Active second stage no more than 2 hours in
primiparous and 60 minutes in multiparous
woman.
7 . Spontaneous vaginal delivery.
8 . Third stage lasting no more than 30 minutes with
active management .
9 . Healthy mother and baby


رفعت المحاضرة من قبل: Hatem Saleh
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