1. Physiology of labor and pain
pathways
Sileshi A.
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2. THEORIES:
Direct pressure exerted on cervix by fetus.
 Progesterone Withdrawal: ↓ progesterone by placenta &
↑ prostaglandins in chorioamnion results in ↑ uterine
contractions.
 Oestrogen Stimulation: ↓ progesterone allows oestrogen
to ↑ contractile response of uterus.
 Fetal Cortisol: Changes biochemistry of fetal membrane:
↓ progesterone & ↑ prostaglandin in placenta.
 Distension: uterine muscles stretch causing ↑
prostaglandin.
 Amniotic membranes (sac) converts arachidonic acid →
Prostaglandin  uterine contractility.
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3. Premonitory signs of labour: weeks
before real labour
 Lightening: Fetus settles into pelvic cavity.
 Braxton-Hicks: Irregular intermittent
contractions; “false labor”.
Cervical changes: cervix effaces [thins] & dilates
slightly
 Baby's head in pelvis pushes against cervix
causing relaxation and effacement.
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4. Signs True Labor: closer to time of
delivery
 Uterine Contractions: regular & frequent compared to
Braxton-Hicks
 Which becomes stronger with time.
 Bloody Show: pink tinged secretions due to softening
cervix.(aka mucous plug)
 Rupture of Membranes: (ROM) Labour in 24 hrs.
Multiparas sooner.
 Clear/odorless.
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5. Difference Between True & False Labor
True Labor
Contractions occur at
regular intervals.
Intervals (b/n conxn.)
gradually shorten.
Intensity gradually
increases.
Discomfort is in the back
and abdomen.
Cervix dilates.
Discomfort is not stopped by
False Labor
Contractions occur at
irregular intervals.
Intervals remain long.
Intensity remains
unchanged.
Discomfort is chiefly in the
lower abdomen.
Cervix does not dilate.
Discomfort usually is
relieved by sedation.
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6. Stages of labor
3/4
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7. First Stage
 Start of regular uterine contractions until the
completion of cervical dilation(=10cm)
 ~ 6-18 hrs. primapara; and 2-10 hrs. multipara.
3 phases : latent, active and transition
Latent phase:- the period between the onset and
the point at which a change in the slope of cervical
dilatation is noted.
Dilation 0-3 cms. Contx.’s mild/irregular.
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8. Cont.
Active phase:- phase of a rapid acceleration of
cervical dilatation (begins @ 3cm)
4-7 cms. Contx.’s 5-8 min. apart. Lasts 45-60 sec;
moderate - strong intensity.
Transitional: Dilation 8-10 cms. Contx.’s 1-2 min.
apart; 60 –90 sec.; strong intensity.
 No pushing until fully dilated.
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9. Second stage
Delivery of infant:
 up to 1 hr. or ~ 20 contx’s – primip.
 20 min. or ~ 10 contx’s in multip. Can last up to 3 hrs.! Esp.
in case of EPA
Cardinal movements occur here.
 Most difficult & uncomfortable part of labor.
 Strong urge to push & bear down as infant passes through
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10. Third Stage
Delivery of placenta ~ 5 - 30 min.
 Separation should be automatic [uterus contracts &
mum bears down]
 Manual presses on contracted uterus. “ Crede’s
Maneuver”
 Syntocinon  placenta delivered to avoid retained
placenta.
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11. Fourth stage
 Placenta out; mother recovers.
 Lasts ~ 1 hr. unless complications arise.
 Then patient is transferred to postnatal unit.
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12. Assessing Progress of Labor
 Dilation: 0–10 cm. [opening cervix]
 Effacement: 0 –100 % [thinning cervix]
 Station: Relationship of presenting part to pelvic ischial spines midway in pelvic cavity.
 “0 ” station aka “engaged”.
 -1 to -5 above “0”
 +1 to +5 (outlet) below “0”
 +4/+5: baby's head out.
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13. Cont.
The progress of labor may be abnormal and can
be classified as a
Slow latent phase,
Arrest of active phase, and
Arrest of descent.
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14. 14

15. Mechanism of Labour
Passage of fetus through birth canal involves position
changes called Cardinal Movements of Labour:
 Engagement: presenting part enters midpoint of pelvis at
ischial spines.
 Descent: downward movement through pelvic inlet
through dilated cervix, reaches posterior vaginal wall.
Mum feels like pushing. Widest part [head] passes through
pelvis.
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16. Cont.
 Internal Rotation: occiput in diagonal position &
rotates towards face down position (OA) (occurs
as body parts press on bony pelvic structures)
 Extension: top of head delivered & extends as
face & chin are delivered.
 External Rotation: head rotates back to previous
lateral position. Rest of body is delivered.
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17. Factors affecting labour process:
3 Ps
Passenger
Passageway
Powers
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18. Passenger: [infant]
A. Fetal head: widest part of body; most difficult to
pass through vaginal canal;
 Passage depends on bones, sutures, fontanelles.
 Cranium - 8 bones meet @ suture lines
 Cranial bones move & overlap, allows skull to pass
thru birth canal.
 Fontanelles: soft spaces created by junctures of
suture lines - covered by membranes; compress
during delivery to aid in passage of fetus.
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19. Cont.
 Skull widest @ antero-posterior diameter than @ transverse
diameter.
 Antero-posterior diameter measures differently @ different
locations.
 Occipitomental diameter- widest - measured from chin to
posterior fontanelle = 13.5 cm
 Smallest diameter - lower occiput to anterior fontanelle
(suboccipitobregmatic) = 9.5 cm
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20. Cont.
B. Fetal Attitude: degree of flexion of fetal head.
Complete flexion: allows smallest diameter of
skull to pass through pelvic cavity. Best position!
Moderate flexion: head less flexed making
diameter wider.
 Poor flexion: brow or face presentation; presents
presents skull diameter too wide making delivery
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21. Cont.
C. Fetal lie: relationship of long axis of fetus
[spine] to long axis of mother:
1. Longitudinal – vertex/breech; vertical in
relation to mum; ~ 99%.
2. Transverse – horizontal in relation to mum; < 1
%.
3. Oblique - diagonal
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22. Cont.
D. Fetal presentation: part of fetal head
enters pelvis;
1. Cephalic 95.5%
2. Breech 3.5%
3. Face 0.3%
4. Shoulder 0.4% [transverse lie]
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23. Cont.
E. Fetal position: “occiput is landmark”
Presenting part [occiput, mentum, sacrum]
Landmark is anterior, posterior, transverse in
relation to mother’s spine.
 Occiptito-anterior (OA) back of head against
symphysis pubis & face towards spine.
 Occipito-posterior (OP) Back of head = mother’s
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24. Passageway:
Refers to fetus passing through uterus, cervix, vaginal canal.
Single most important determinant to mechanism of labor.
A. Shape of pelvis:
 1. Gynaecoid – 50% of women; rounded, oval shape;
easy vaginal delivery; considered “normal female
pelvis”
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25. Cont.
 2. Android – 20 % of
women; vaginal delivery
difficult; prob. C/S;
“true male pelvis”
 3. Anthropoid – oval;
assisted vaginal birth
usually with forceps;
20-25%
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26. Cont.
 4. Platypelloid – < 5 %
of women; flattened
pelvis; vaginal delivery
difficult
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27. Cont.
B. Structure of Pelvis
 False Pelvis: Outer - broader. Hip bones.
 True Pelvis: Internal – narrower. Holds bladder, rectum, &
reproductive Organs.
 True pelvis - has 3 parts - inlet, midpelvis, outlet
[Most important in childbirth]
 Contractions of the pelvic inlet, the midpelvis, the
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28. Cont.
Powers:
 Uterine contx’s: primary force moving fetus
thru maternal pelvis during 1st stage of
Maternal Efforts: woman adds voluntary
pushing force to force of contx.’s during 2nd
of labor to propel fetus thru pelvis.
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29. Physiology of pain in labor and
Neural pathways
Perception of pain by the parturient is dynamic
process
It Involves both peripheral and central mechanisms
Many factors affect degree of pain experienced by
woman including:-
Psychological preparation,
Emotional support during labor,
Past experiences,
The patient's expectations of the birthing
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30. Cont.
1st stage of labor – mostly visceral
◦ Dilation of the cervix and distention of the lower
uterine segment
◦ Dull, aching and poorly localized
Slow conducting, visceral C fibers, enter spinal cord at
T10 to L1 to synapse in the dorsal horn.
The chemical mediators involved are bradykinin,
leukotrienes, prostaglandins, serotonin, substance P
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31. Cont.
2nd stage of labor – mostly somatic
◦ Distention of the pelvic floor, vagina and
perineum stimulation of pudendal nerve.
◦ Sharp, severe and well localized
Rapidly conducting A-delta fibers, enter spinal cord
at S2 to S4 impulses pass to dorsal horn cells and
finally to the brain via the spino-thalamic tract.
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32. Neural pathways 32

33. Physiological response to labor pain
System Response to pain
CVS Pain increases catecholamine level increase in
contractility and SVR, all of which increases
oxygen demand
Placenta Pain increases catecholamine levels
of umbilical vessels and consequently reducing
placental blood flow
Respirat
ry
Pain increases MV maternal hypocapnoea
respiratory alkalosis shifts the oxy-hgb disso.
Lt decreased O2 offloading to the fetus
GIT Pain reduces gastric emptying increasing risk of
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34. References
Williams obstetrics 23rd edition
Millers anesthesia 7th edition
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