Normal mechanism of labour and factors influencing it.

1. Normal Mechanism of
Labour
DR BISMA AKHTAR
RESIDENT OBGYN

2. Definition of labour
 Presence of strong, regular, painful contractions
with associated cervical changes including
effacement, dilatation or both.
 This physiological process will result in delivery of
baby, placenta and membranes and signals for
lactation to begin.

3. Mechanism of labour
 It refers to the series of passive adaptive
positional movements that the fetus undergoes
during its passage through the birth canal to
accommodate itself in the maternal pelvis.
 We describe normal mechanism of labour for
vertex presentation and gynaecoid pelvis.

4. Passive Cardinal Movements
 Engagement
 Descent
 Flexion
 Internal rotation
 Extension
 Restitution
 External rotation
 Delivery of shoulders and fetal body

5. Engagement
 It is when the babys head is in line with the ischial
spine.
 The fetal head normally enters the pelvis in transverse
position, due to widest transverse diameter of pelvic
inlet I.e 13.5cm.
 Engagement is said to have occurred when the widest
part of presenting part has passed successfully through
the inlet.

6.  Number of fifths of fetal head palpable abdominally is
used to describe whether engagement has taken place.
 If more than two-fifth of fetal head is palpable
abdominally, head is not yet engaged.
 In nulliparous, engagement occurs prior to labour.

9. Descent
 It is downward passage of presenting part of
fetus through the maternal pelvis. It occurs
intermittently with contractions.
 It is mediated by uterine contractions,
amniotic fluid pressure and abdominal muscle
contractions.

10. Flexion
 In the narrow midpelvis, flexion occurs due to
resistance from soft tissue of pelvis. It
happens during first stage of labour.
 It facilitates the shortest AP diameter
(suboccipito bregmatic of head to be
presented at pelvic outlet. I.e. 9.5cm.

13. Internal rotation
 As the head descends, presenting part usually is in
transverse position.
 On reaching the sloping gutter of the Levator ani
muscles, leading part i.e occiput is rotated 45 degrees
to come anteriorly, under the symphysis pubis.
 It bring AP diameter of fetal head in line with AP
diameter of pelvic outlet.

15. If engages in OP position:
 Long internal rotation leading to prolonged
labour
 Face to pubes delivery
 Due to extension of fetal head. This will lead
to obstructed labour, need for instrumental
delivery or C section.

16. Extension
 Well flexed head extends and occiput escapes
from under neath the symphysis pubis and
distends the vulva, known as crowning of the
head.
 Head extends further, bregma face and chin
appear in succession over the posterior vaginal
opening.

18. Restitution
 As occiput crosses perineum, head aligns
itself with the shoulders, which have entered
the pelvis in oblique position.
 It happens along one eighth of a circle.

20. External rotation
 In order to be delivered, shoulders have to
rotate into AP plane (widest diameter at the
outlet).
 When it happens, head rotates a further one
eighth of a circle. This is external rotation.

21. Delivery of shoulders and fetal
body
 The anterior shoulder is under symphysis pubis
and delivers first. Followed by posterior.
 Rest of fetal body is delivered with posterior
shoulder guided over the perineum by gentle
upward traction, delivering the baby on to
maternal abdomen.

24. Factors influencing Normal
mechanism of labour
3Ps
 POWER
 PASSAGE
 PASSENGER

25. Power
It refers to forces that result in passage of fetus
through the birth canal. The are,
 Uterine contractions In first stage and passive
phase of second stage of labour. Characterized by
frequency, intensity and duration.
 Maternal efforts of pushing in Active phase of
second stage of labour

26. The Passage
It refers to birth canal itself, comprising of
 Bony pelvis (Gynaecoid is the most favourable
one)
 Muscles of pelvic floor
 Soft tissues of perineum

27. The passenger
It refers to fetus in terms of its
 Size (Small,average,large)
 Presentation (vertex, face,brow,breech)
 Position (OA,OP)

29.  When 3Ps are favourable normal labour is
likely to ensue. Resulting in unassissted or
spontaneous vaginal birth.

30. THANK YOU.