1. Mechanism of normal labor
Dr ERMIAS
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2. LABOUR
• Physiologic process by which a fetus is
expelled from the uterus to the outside world.
• An increase in myometrial activity
• precisely, a switch in the myometrial
contractility from ”contractures” (long lasting,
low frequency activity) to
“contractions”(frequent, high intensity, high
frequency activity) resulting in effacement and
dilatation of uterine cervix.
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3. • Mean duration of human singleton pregnancy
is 280days(40 weeks) from the last menstrual
period.
• “term pregnancy” refers to period from
37.0weeks to 42.0 weeks of gestation
• “pre_term pregnancy”: onset of labour prior to
36 completed weeks of gestation
• “post_term pregnancy”: pregnancy continuing
beyond 42.0 weeks of gestation
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4. Diagnosis
clinical diagnosis
classic diagnosis of labor includes
• Regular painful uterine contractions
• Progressive cervical effacement and dilatation
• Show + rupture of the fetal membranes
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5. Physiology of normal labour at term
• factors responsible for initiation and
maintenance of labor at term are not well
defined.
• Theories
• progesterone withdrawal
• oxytocin induction
• fetal
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6. Phases of parturition
Regulation of uterine activity during pregnancy is divided into
four distinct physiologic phases.
• Phase 0: state of functional uterine
quiescence
•
Phase 1: phase of uterine
activation (awakening)
• Phase 2: phase of uterine stimulation
•
Phase 3: phase of uterine involution
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7. Mechanisms of labour
• The ability of the fetus to successfully
negotiate the pelvis during labour and
delivery is dependent on the complex
interactions of three variables
–The power
–The passenger
–The passage
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8. The power
• Force generated by uterine musculature
• Characterized by frequency, amplitude(intensity),and
duration.
• Assessment may include, simple observation, manual
palpation,external objective assessment
techniques(external tocodynamometry), or direct
measurement of intrauterine pressure using(internal
manometry or pressure transducers)
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9. ……..CONT’D
“adequate” uterine contractions
• progressive cervical effacement and dilatation
• 3_5 contractions in 10minutes or
7contractions in 15 minutes
• 200_250montevidio units
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10. the passenger
fetal variables that influence the course of labor and delivery
• Absolute fetal size (macrosomia >4500gms)
• Multifetal pregnancy
• Lie: the longitudinal axis of the fetus in relation to
the longitudinal axis of the uterus
Either longitudinal, Transverse or oblique
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11. ….the passenger
• presentation: the fetal part that directly
overlies the pelvic inlet
cephalic, breech, shoulder, funic, compound
• Malpresentation is any presentation that is
not cephalic with occiput leading, accounting
for 5% of all term labor
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12. …passenger
• attitude: position of head with regard to the
fetal spines
• position: the relationship of a nominated site
of presenting part to the
denominating location of the internal
pelvis
• Position can be most accurately assessed on
transvaginal examination.
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13. Malposition is any position in labor
which is not ROA,OA or LOA
• presentations
• Occiput
• Breech
• face
• shoulder
• Nominated sites
»vertex
»Sacrum
»Mentum
» Acromium
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14. N.B in both classifications the midpoint (0 station) the
plane of the maternal ischial spines
• station : measure of descent of the
presenting part of the fetus through the
birth canal.
• old classification - by subjective arbitrary assignment of
seven stations(-3 to +3)
• new classification attempts to quantitate in centimeter the
distance of the leading bony edge from the ischial spines of
the maternal pelvis(-5 to +5).
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15. The passage
consists of
• the bony pelvis (composed of the sacrum, ilium,ischium and
pubis) and
• the resistance by the soft tissues of the pelvis.
The pelvis has a series of planes broadly classified into
• pelvic inlet,
• Mid-pelvis cavity and
• outlet.
• The shape of the female bony pelvis can be classified as:
gynecoid, anthropoid, android and platypeloid
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16. Stages of labor
• three stages,
First stage: the interval between the onset of labor and
full cervical dilatation
has been subdivided into three phases according to
the rate of cervical dilatation
• latent phase : the interval between the onset of
labor and a point at which a change in the slope of
the rate of cervical dilatation in noted usually 2-3cm.
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17. …First stage of labor
• active phase : associated with a
greater rate of cervical
dilatation and usually
begins at 3-4cm
interval between 3-10cm of
cervical dilatation
• descent phase : coincides with the
second stage of labor
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18. • second stage : interval between full
cervical dialitation(10cm) and
delivery of an infant. Characterized by descent of the
presenting part through the maternal pelvis
culminating in expulsion of the fetus.
• Third stage :the delivery of the placenta and
fetal membranes usually lasts less than
30 minutes.
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19. Cardinal movements in labor
• the changes on position of fetal head during its
passage through the birth canal.
• Because of the asymmetry of the shape of both the
fetal head and the maternal bony pelvis, such
rotations are required for the fetus to successfully
negotiate the birth canal.
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20. seven discrete movements of the fetus
1. engagement : passage of the widest diameter of the
presenting part to a level below the plane of the
pelvic inlet. The sagital
suture is frequently deflected either posteriorly to
the promontory or anteriorly to the symphysis
pubis such lateral deflection the fetal head is known
as anterior and posterior asynclitism.
2. Descent : downward passage of the presenting part
through the pelvis.
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Descent: As the fetal head engages and descends, it assumes an occiput transverse
position because that is the widest pelvic diameter available for the widest part of the
fetal head.

22. …cardinal movements
3. Flexion : flexion of the fetal head occurs passively as
the head descends due to the shape of the bony
pelvis and the resistance offered by the soft
tissues..
4. Internal rotation : rotation of the presenting part
from its original position (usually transverse with
regard to the birth canal) to the anterior position
as it passes through the pelvis.
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25. …cardinal movements
5.Extension : occurs once the fetus has reached the
level of the interoitus, bringing the base of occiput in
contact to the inferior margin at the symphysis
pubis.
6.External rotation (restitution) : is return of the fetal
head to the correct anatomic position in relation to
the fetal torso.
7.Expulsion : delivery of the rest of the fetal body.
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28. • Cardinal movements are better
observed by
partogram
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