This document discusses the physiology of labor and pain pathways. It covers the stages of labor from early signs through delivery. The four stages are outlined as well as factors that can affect the labor process including the passenger (fetus), passageway (pelvis), and powers (uterine contractions and maternal efforts). Pain in labor is described as having both peripheral and central mechanisms. Visceral pain occurs in the first stage as the cervix dilates while somatic pain in the second stage results from pressure on the pelvic floor. The neural pathways and physiological responses to labor pain are also summarized.

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 →
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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
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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.
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
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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”
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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
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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,
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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,
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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
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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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