1

Physiology of labor and pain pathways

Sileshi A.

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.

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.

Cervix in posterior position.

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.

Green/brown danger sign

Meconium aspiration distress/infection Immediate medical

attention.

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 sedation.

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.

6 Stages of labor 3/4

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.

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.

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 vagina & rectum

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.

If no spontaneous delivery of placenta,

manually removed.

Antibiotics

11 Fourth stage

Placenta out; mother recovers.

Lasts ~ 1 hr. unless complications arise.

Then patient is transferred to postnatal

unit.

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.

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.

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.

Flexion: pressure from pelvic floor causes head to

flex towards chest; chin touches chest.

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.

17

Factors affecting labour process: 3 Ps

Passenger

Passageway

Powers

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.

“Molding” of infant head.

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

Complete flexion allows smallest diameter of fetal

skull to enter pelvis most easily.

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 skull diameter too wide making

delivery difficult.

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

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]

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 spine; painful contxs.

Transverse (T) = fetus sideways.

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”

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%

26 Cont.

4. Platypelloid – < 5 % of women; flattened pelvis; vaginal delivery difficult

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 pelvic outlet, or a generally contracted

pelvis Fetopelvic Disproportion

28 Cont.

Powers:

Uterine contx’s: primary force moving fetus

thru maternal pelvis during 1st stage of labor.

Maternal Efforts: woman adds voluntary

pushing force to force of contx.’s during 2nd

stage of labor to propel fetus thru pelvis.

29 Physiology of pain in labor and Neural pathways

Perception of pain by the parturient is dynamic processIt Involves both peripheral and central mechanismsMany factors affect degree of pain experienced by

woman including:-Psychological preparation, Emotional support during labor, Past experiences, The patient's expectations of the birthing process,

and Induction or augmentation of labor with oxytocin.

30Cont.

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 and lactic acid

31Cont.

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.

32 Neural pathways

33Physiological response to labor pain Syste

m Response to pain

CVS Pain increases catecholamine level increase in HR, contractility and SVR, all of which increases myocardial oxygen demand

Placenta

Pain increases catecholamine levels vasoconstriction of umbilical vessels and consequently reducing placental blood flow

Respiratory

Pain increases MV maternal hypocapnoea respiratory alkalosis shifts the oxy-hgb disso. Curve to Lt decreased O2 offloading to the fetus

GIT Pain reduces gastric emptying increasing risk of aspiration

34 References

Williams obstetrics 23rd edition Millers anesthesia 7th edition