Labor, Obstetrics

1. MECHANISM OF LABOR
NIVETHA C

2. DEFINITION
The series of movements that occur on
the head in the process of adaptation
during its journey through the pelvis is
called mechanism of labor

5. FETAL LIE
Relation of the long axis of the fetus to the long
axis of the maternal spine
Longitudinal – Parallel
Transverse – Perpendicular
Oblique- 45˚

6. FETAL PRESENTATION
• The part of the fetus which occupies
the lower pole of the uterus
• Cephalic
• Podalic
• Shoulder
• Compound

9. PRESENTING PART
• Part of the presentation which
overlies the internal os
• Cephalic presentation
1) Vertex
2) Brow
3) Face

11. ATTITUDE
• Relation of the different parts of the
fetus to one another
• Universal flexion
 Head, trunk and limbs Attitude of flexion
on all joints
 Ovoid mass

13. DENOMINATOR
•Bony fixed point on the presenting part
used for describing the position in relation
to the various quadrants of maternal pelvis

14. POSITION
•Relation of the denominator to the
different quadrants of the pelvis
•Pelvis8 segments of 45˚

16. MECHANISM
•The head enters more commonly through the
available transverse diameter and to lesser
extent through oblique diameters
•AP diameter- suboccipitibregmatic (9.5cm)
-suboccipitofrontal(10cm)
•Transverse diameter- biparietal(9.5cm)
•Occipitolateral position is more common,so the
mechanism in such position will be described

17. PRINCIPAL MOVEMENTS
1.Engagement
2.Descent
3.Flexion
4.Internal rotation
5.Crowning
6.Extension
7.Restitution
8.External rotation
9.Expulsion of the trunk

18. ENGAGEMENT
•When BPD had crossed the brim
•Synclistism – sagittal suture corresponds to TD
•Asynclitism
1. Anterior
2. Posterior
•Mild degrees of asynclitism are common but
severe degrees indicate cephalopelvic
disproportion
•In primigravidae – before onset of labor
•In multiparae- late first stage with ROM

20. DESCENT
• It is a continuous process
• Slow or insignificant in 1st stage but pronounced in
2nd stage
• It is completed with expulsion of fetus
• In primigravidae – no descent in 1st stage
• In multiparae- it starts with engagement
Factors facilitating descent
1. Uterine contraction and retraction
2. Bearing down efforts
3. Straightening of the ovoid fetal especially
after rupture of membranes

21. FLEXION
• Some degree of flexion occurs at the beginning of
labor
• Complete flexion – head meets the resistance of the
birth canal during descent
• Resistance is offered by
1. Unfolding cervix
2. Walls of pelvis
3. Pelvic floor
• It is essential for descent as it reduces the shape and
size of the plane of advancing diameter of head
• It is explained by the twoarm lever theory

22. INTERNAL ROTATION
• It is a movement of great importance without which
there will be no further descent
• Anterior rotation of occiput are explained by the
following theories
1. Law of pelvic floor( Hart’s rule)
2. Law of pelvic shape
3. Law of unequal flexibility
• The level at which internal rotation occurs is
variable
• More common at pelvic floor
• More favourable at cervix but less frequent
occurrence

23. TORSION OF THE NECK
• It is an inevitable phenomenon during rotation of head
• If shoulders remain in AP diameter, the neck has to sustain
torsion of 2/8th of a circle corresponding with the same
degree of anterior rotation of the neck
• But neck fails to withstand resulting in simultaneous
rotation of shoulders in same direction to extent of 1/8th of a
circle
• In left occipitolateral position – left oblique diameter
• In right occipitolateral position –right oblique dimeter
• In oblique occipitoanterior position –no movement of
shoulder

24. CROWNING
• After internal rotation further descent occurs until
subocciput lies underneath the pubic arch
• The maximum diameter of the head (biparietal
diameter) stretches the vulval outlet without any
recession of the head even after the contraction is
called crowning of the head

25. EXTENSION
• Delivery of the head takes place through ‘couple of force
theory’
• The driving force pushes the head in downward direction
• Resistance is offered by the pelvic floor in upward and
forward direction
• Upward and downward force neutralize
• Remaining forward thrust helps in extension
• The successive parts of fetal head to be born through the
stretched vulval outlet are vertex ,brow and face
• Release of chin through anterior margin of the stretched
perineum head drops down  chin comes in close
proximity to the maternal anal opening

26. RESTITUTION
• It is the visible passive movement of the head due to
untwisting of the neck sustained during internal
rotation
• Rotating the head through 1/8th of a circle in the
direction opposite to that of internal rotation
• The occiput points to the maternal thigh of the
corresponding side to which it originally lay

27. EXTERNAL ROTATION
• It is the movement of rotation of the head visible
externally due to internal rotation of the
shoulders
• The occiput points directly towards the maternal
thigh corresponding to the side to which it
originally directed at the time of engagement

28. BIRTH OF SHOULDERS AND TRUNK
• After the shoulders are positioned in AP diameter of
the outlet further descent takes place until the
anterior shoulder escape below the symphysis pubis
first
• By a movement of lateral flexion of the spine ,the
posterior shoulder sweeps over the perineum
• Rest of the trunk is expelled out by lateral flexion

30. CLINICAL COURSE OF FIRST STAGE OF LABOR
• Intermittent painful uterine contraction
followed by expulsion of blood stained mucus
per vagina
1. Pain
2. Dilatation and effacement of cervix
3. Status of the membranes
4. Maternal effect
5. Fetal effect

31. PAIN
• Pain are felt more anteriorly with simultaneous
hardening of the uterus
• In early first stage
Interval15 to 30 minutes
Duration30 seconds
• In late first stage
Interval3 to 5 minutes
Duration45 seconds
• In normal labor pain is usually felt shortly after
the uterine contraction begin and pass off before
complete relaxation of the uterus

32. DILATATION AND EFFACEMENT
• Cervical dilatation relates with dilatation of external os
• Effacement is determined by the length of the cervical canal in the vagina
• In primigravida cervix may be completely effaced
• In multiparae dilatation and taking up occur simultaneously which are
more abrupt following the rupture of the membranes
• The anterior lip of the cervix is the last to be effaced
• The first stage is completed only when the cervix is completely retracted
over the presenting part during contractions
• Cervical dilatation is expressed in terms of fingers -1,2,3 or fully dilated or
in centimeters
• Effacement in terms of percentage
• Rim is used when the depth of the cervical tissue surrounding the odds is
about 0.5 to 1 cm

33. PARTOGRAPH

35. • Cervical dilatation is a sigmoid curve
• First stage has got two phases
1. Latent phase
2. Active phase
Latent phase: period between the onset of true labor
and the point when the cervical dilatation becomes 3-4
cm
• Duration in primigravida – 20 hours
• Duration in multipara – 14 hours
• Average cervical dilatation – 0.35 cm/ h

36. • Active phase:
1. Acceleration phase with cervical
dilatation of 3-4 cm
2. Phase of maximum slope of 4-9 cm
dilatation
3. Phase of deceleration of 9-10 cm
• Dilatation of the cervix at the rate of 1 cm/ hr in
primigravida
• 1.5 cm/hr in multigravida beyond 4 cm
dilatation

37. STATUS OF THE MEMBRANES
• Membranes usually remain intact until full dilatation
of the cervix our sometimes beyond in the 2nd stage
• Early rupture – rupture any time after the onset of
labor but before full dilatation of cervix
• Premature rupture – before the onset of labor

38. MATERNAL EFFECT
• General condition – unaffected
• Transient fatigue following strong contraction
• Pulse rate – increased by 10 to 20 bpm during
contraction, settles down in between
contraction
• Systolic blood pressure raised by 10 mm Hg
• Temperature - unchanged

39. FETAL EFFECT
• As long as membranes are intact – no adverse
effects
• During contraction there may be slowing of
fetal heart by 10- 20 bpmwhich soon returns to
normal rate of about 140 per minute

40. CLINICAL COURSE OF SECOND STAGE
Second stage begins with full dilatation of the
cervix and ends with expulsion of the fetus
PAIN
• Intensity of pain increases
• Interval – 2 to 3 mins
• Lasts for 1 to 1.5 mins

41. BEARING DOWN EFFORTS
• It is the additional voluntary expulsive efforts that appear during
the second stage of labor ( expulsive phase)
• It is initiated by the nerve reflux(Ferguson reflux) set up due to
stretching of the vagina by the presenting part
• In majority this expulsive effort start with full dilatation of the
cervix
• Exert downward pressure
• Sustained pushing beyond the uterine contraction is discouraged
• Premature bearing down efforts may suggest uterine dysfunction
• Slowing of the FHR during pushing

42. MEMBRANES STATUS
• It may rupture with gush of liquor per vaginum
• It may be delayed till the head bulges out through
the introitus
• Spontaneous rupture may not take place at all
allowing the baby to be “born in a caul”

43. DESCENT OF THE FETUS
• Abdominal findings – progressive descent of the
head, assessed in relation to the brim, rotation of the
anterior shoulder to the midline and change in
position of the FHS (shifted downward and medially)
• Internal examination reveals the descent of head in
relation to the ischial spines and gradual rotation of
head evidenced by position of the sagittal suture and
occiput in relation to quadrants of the pelvis

44. Fifth formula
• Progressive descent of the head can be accessed by
estimating the number of fifths of the head above the
pelvic brim
• Amount of head feels suprapubically in finger breath is
assessed by placing the radial margin of index finger
above the symphysis pubis until the groove of the neck
is reached

47. Advantages over station of the head in relation to ischial
spines
• It excludes the variability due to car and molding buy
different depth of the pelvis
• The assessment is quantitative and can be easily
reproduced
• Repeated vaginal examination are avoided

49. Vaginal signs
• As the head descends down, it distends the perineum, the vulval
opening looks like a slot through which the scalp hair is visible
• It becomes circular
• The adjoining sphincter is stretched and stool comes out during
contraction
• The head recedes after the contraction passes off but is held in
advance because of retraction
• The maximum diameter of the head stretches the vulval outlet and
there is no recession even after contraction passes off(Crowning)
• Further pain and bearing down efforts to expel the shoulders and
trunk followed by gush of liquor

50. MATERNAL SIGNS
• Exhaustion
• Respiration is slowed down with increased
perspiration
• Face congested with engorged neck veins prominent
• Sigh of relief after expulsion of fetus
FETAL EFFECTS
• Slowing of FHR during contraction is observed which
comes back to normal before the next contraction

51. • Third stage includes separation,descent and expulsion of the
placenta with its membranes
• PAIN – no pain, intermittent discomfort
BEFORE SEPERATION –
Per abdomen: uterus discoid in shape,firm in feel and non
ballottable
Fundal ht reaches below the umbilicus
Per vaginum: slight trickling of blood and length of the umbilical
cord as visible from outside remains static
CLINICAL COURSE OF THIRD STAGE

52. AFTER SEPARATION
Per abdomen:
1. Uterus becomes globular, firmand ballottable
2. The fundal height is slightly raised
3. Slight bulging in the suprapubic region due to
distension of the lower segment by the separated
placenta
Per vaginum:
1. Slight gush of vaginal bleeding
2. Permanent lengthening of the cord is established

53. EXPULSION OF PLACENTAAND MEMBRANES
• This is achieved either by voluntary bearing down
efforts or more commonly aided by manipulative
procedure
• The afterbirth delivery is soon followed by slight to
moderate bleeding amounting to 100 to 250 ml
MATERNAL SIGNS: Chills, occasional shivering,
slight transient hypotension