Mechanism

1. MECHANISM OF LABOUR (NORMAL &
ABNORMAL)

2. Lie, presentation, attitude, &position
FETAL LIE
 The relation of the long axis of the fetus to that of the mother
 Longitudinal lie is found in 99% of labours at term
 Predisposing factors for transverse lie/oblique lie 
multiparity, placenta previa, hydramnious, & uterine
anomalies
FETAL PRESENTATION
 The presenting part is the portion of the body of the fetus
that is foremost in the birth canal
 The presenting part can be felt through the Cx on vaginal
examination
 Longitudinal lie  cephalic presentation
breech presentation
 Transvrse lie  shoulder presentation

3. Lie, presentation, attitude, &position
CEPHALIC PRESENTATION
 Head is flexed sharply  vertex / occiput presentation
 Head is extended sharply  face presentation
 Partially flexed  bregma presenting (sinciput presentation)
 Partially extended  brow presentation
BREECH PRESENTATION
 Frank breech
 Complete breech
 Footling breech
ATTITUDE
 Posture of the fetus folded on itself to accommodate the shape of the
uterus
 Flexed head, thighs, knees &feet
 The arms crossed over the chest
 Face presentation extended concave contour of the vertebral column

4. .
'
"
'
I
!
1
1
1
A
B
i
i
F
c
D
Longitudinal lie. Cephalic presentation. Differences in attitude of fetal
body,
Note changes in fetal attitude in relation to fetal vertex as the fetal head
becomes less flexed.
(A) vertex (B) sinciput (C) brow (D) face

5. Longitudinal lie. Frank breech
presentation.
I
I
Longitudinal lie. Complete breech
presentation.

6. Longitudinal lie. Incomplete, or footling,
breech presentation.

7. POSITION
The relation of an arbitrary chosen point of the fetal
presenting part to the Rt or Lt side of the maternal birth
canal
The chosen point
 Vertex presentation  occiput
 Face presentation  mentum
 Breech presentation Sacrum
Each presentation has two positions Rt or Lt
Each position has 3 varieties : Ant, transverse, post
OA
OP
LOT
ROT
LOA
ROA
LOP
ROP

8. ~
'
t
J
LONGITUDINAL LIE VERTEX PRESENTATION
LOA LOP

9. s
A
Longitudinal lie. Vertex presentation
A. Right occiput posterior (ROP) Right occiput transverse (ROT)

10. S
f
a
!
w
C
c
h
t
p
(
f
i
t
t
b
f
e
w
c
a
l
t
a
Right occiput anterior (ROA).

11. FREQUENCY OF VARIOUS PRESENTATIONS &
POSITIONS AT TERM
 Vertex  96%
2/3 Lt
1/3 Rt
 Breech  3.5%
 Face 0.3%
 Shoulder 0.4%

12. Longitudinal lie. Face presentation. Left and right
anterior and ri posterior positions.
Rt mento-post
Rt mento-ant
Lt mento-ant

13. ~
Longitudinal lie Breech presentation LSP

14. Transverse lie. Right acromiodorsoposterior position (RADP). The
shoulder of the fetus is to the mother's right, and the back is posterior.

15. MECHANISM OF LABOUR WITH OCCIPUT
PRESENTATIONS
THE CARDINAL MOVEMENTS OF LABOUR
1-ENGAGEMENT
The greatest transverse diameter BPD passes through the
pelvic inlet
It may occur in the last few weeks of pregnancy or only in
labour especially in multipara
The fetus enters the pelvis in transverse or oblique diameter
 LOT  40%
 ROT 20%
 OP 20% ROP >LOP
 ROA / LOA 20%

16. THE CARDINAL MOVEMENTS OF LABOUR
 Asynclitism
The sagittal sutures of the head deflects ant towards the
symphysis pubis or post towards the sacrum
2-DESCENT
 In nullipara engagement takes place before the onset of
labour & further descent may not occur till the 2nd stage
 In multipara descent begins with engagement
 It is gradually progressive till the fetus is delivered
 It is affected by the uterine contractions & thinning of the
lower segment

17. Anterior asynclitism
Naegele's obliquity
Normal synclitism Posterior asynclitism
Litzmann's obliquity Ear
presentation

18. 3-FLEXION
 The descending head meets resistance of pelvic floor, Cx
& walls of the pelvis   flexion
 The shorter suboccipito-begmatic is substituted for the
longer occipito-frontal

19. Lever action producing ftexion of the head; conversion
from occipitofrontal to suboccipitobregmatic diameter
typically reduces the anteroposterior diameter from
nearly 12- to 9.5 cm.

20. A
c
Four degrees of head
flexion. Indicated by the
solid line the
occipitomental diameter;
the broken line connects
the center of the anterior
fontanel with posterior
fontanel:
A. Flexion poor.
B. Flexion moderate.
C. Flexion advanced.
D. Flexion complete.
Note that with flexion
complete the chin is on
the chest, and the
suboccipitobregmatic
diameter, the shortest
anteroposterior diameter
of the fetal head, is
passing through the pelvic
inlet.
A
c
D

21. 4-INTERNAL ROTATION
 Turning of the head from the OT position  anteriorly
towards the symphysis pubis ie. Occiput moves from
transverse to ant 45º
 Less commonly OT  posteriorly towards the sacrum
135º
 It is not accomplished till the head has reached the spines
The levator ani muscles form a V shaped sling that tend to
rotate the vertex anteriorly
 It is completed by the time the head reaches the pelvic
floor 2/3 or shortly after ¼

22. EXTENSION
 When the flexed head reaches the vulva it undergoes
extension  the base of the occiput will be in direct contact
with the inferior margin of the symphysis pubis
 Crowning  the largest diameter of the fetal head is
encircled by the vulvar ring
 The head is born by further extension as the occiput,
bregma, forehead, nose, mouth & chin pass successively
over the perineum

23. EXTERNAL ROTATION
RESTITUTION
 After delivery of the head it returns to the position it
occupied at engagement , the natural position relative to
the shoulders (oblique position)Restitution
 Then the fetal body will rotate to bring one shoulder
anterior behind the symphysis pubis ( biacromial diameter
into the APD of the pelvic outlet)
 Restitution is followed by complete external rotation to
transverse position (occiput lies to next to Lt maternal
thigh)
 The ant shoulder slips under the pubis
 By lateral flexion of the fetal body the post shoulder will be
delivered & the rest of the body will follow

24. 3
0
2
2.Engagement;descent, flexion 6. Restitution (external rotation)
3. Further descent, internal rotation
4. Complete rotation, beginning
extension
Cardinal movements in the
mechanism of labor and
delivery, left occiput
anterior position.

25. Mechanism of labor for the left occiput transverse
position, lateral view. Posterior asynclitism (A) at the
pelvic brim followed by lateral flexion, resulting in
anterior asynclitism (B) after engagement, further
descent (C), rotation, and extension (D).
3
0
4
F
t
l
v
b
a
f
s

26. OCCIPUT POSTERIOR POSITION
 Mechanism of labour is identical to OT & anterior varieties
 The occiput rotate to the symphysis pubis through 135º
instead of 90º or 45º
 If rotation does not occur direct occiput post or
Partial rotation transverse arrest

27. Mechanism of labor for right occiput
posterior position, anterior rotation.
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.
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