Normal labour and mechanisms of labour

1. MECHANISM OF LABOUR
MODERATOR:DR.SAROJ(3rd YEAR RESIDENT)
DR.RAVI SAH (CONSULTANT)
LECTURER IN OBSTETRICS AND GYNAECOLOGY
NMCTH,BIRGUNJ
PRESENTER:DR.SURYA PRATAP SINGH RAJPUROHIT

2. Definition of labour:-
• Series of events that take place in the genital organs
in an effort to expel the viable products of
conception (fetus, placenta and the membranes) out
of the womb through the vagina into the outer
world is called Labor
• Definition of delivery:- Is the expulsion or extraction
of a viable fetus out of the womb.

3. • Normal labour :Labour is normal when it is
• 1. Spontaneous in onset and at term
• 2. vertex presentation
• 3. Without undue prolongation
• 4. Natural termination with minimal aids
• 5.without having any complications affecting the health of
mother and/or the baby.

4. Causes of the onset of labour:
• THE PRECISE MECHANISM OF INITIATION OF HUMAN LABOUR IS
STILL OBSCURE,
• Endocrine, biochemical and mechanical stretch pathways were postulated.
1. Uterine distension theory –
• Uterine distension:
• Stretching efect on the myometrium by the growing fetus and liquor amnii can
explain the onset of labor at least in twins or polyhydramnios.
• Uterine stretch increases gap junction proteins, receptors for oxytocin and specifc
contraction associated proteins (CAPs).

5. Fetomaternal contribution:
Fetal hypothalamic-pituitary –adrenal axis prior to onst of labour
Increased CRH and increased release of ACTH
Fetal adrenals increased cortisol secretion
Accelerated production of oestrogen and prostaglandins from the
placenta

6. Estrogen: The probable mechanisms are:
• Increases the release of oxytocin from maternal pituitary.
• Promotes the synthesis of myometrial receptors for oxytocin , prostaglandins and
increase in gap junctions in myometrial cells.
• Accelerates lysosomal disintegration in the decidual and amnion cells resulting in
increased prostaglandin (PGF2α) synthesis
• Stimulates the synthesis of myometrial contractile protein—actomyosin through
cAMP .
• Increases the excitability of the myometrial cell membranes.

7. Prostaglandins:
Prostaglandins are the important factors which initiate and maintain labour.
The major sites of synthesis of porstaglandin are- amnion, chorion, decidual cells
and myometrium
Synthesis is triggered by -rise in estrogen level, glucocoticoids, mechanical
stretching in late pregnancy, increase in cytokines, infection, sepration or rupture of
the membranes.

8. Biochemical Mechanisms involved in the Synthesis of Prostaglandins-
• Phospholipase A2 in the lysosomes of the fetal membranes near term
• esterified arachidonic acid
• formation of free arachidonic acid
• synthesis of prostaglandins through prostaglandin synthetase.
• Prostaglandins (E2 and F2α) diffuse in the myometrium.
act directly at the sarcoplasmic reticulum
• inhibit intracellular cAMP generation → increase local free calcium ions →
uterine contraction.

9. Oxytocin and myometrial oxytocin receptors:
(i) Large number of oxytocin receptors are present in the fundus compared to the
lower segment and the cervix.
(ii) Receptor number increases during pregnancy reaching maximum during labor.
(iii) Receptor sensitivity increases during labor.
(iv) Oxytocin stimulate synthesis and release of PGs (E2 and F2α) from amnion and
decidua.

10. Neurological factor:
Labor may be also initiated through nerve pathways.
• Both α and β adrenergic receptors are present in the myometrium.
• estrogen cause the α receptors and progesterone the β receptors to function
predominantly.
• The contractile response is initiated through the α receptors of the postganglionic
nerve fibers in and around the cervix, and the lower part of uterus.

11. • The basic elements involved in the uterine contractile systems are:
(a) actin
(b) myosin
(c) adenosine triphosphate (ATP)
(d) the enzyme myosin light chain kinase (MLCK)
(e) Ca++

12. Structural unit of a myometrial cell is myofibril which contains the proteins—actin
and myosin.
The interaction of myosin and actin is essential for muscle contraction.
The key process in actin-myosin interaction is myosin light chain phosphorylation.
This reaction is controlled by myosin light chain kinase (MLCK).
Oxytocin acts on myometrial receptors and activates phospholipase C, which
increases intracellular calcium level.
Calcium is essential for the activation of MLCK and binds to the kinase as
calmodulin–calcium complex.
Intracellular calcium levels are regulated by two general mechanisms :
(1) influx across the cell membrane
(2) release from intracellular storage sites.
Progesterone and cAMP promote calcium storage at these sites.

13. • Intracellular calcium,calmodulin ca++-MLCK-phosphorylated myosin +actin
• Causes myometrial contraction.
• Uterine muscles have two types of adrenergic receptors-
1.Alpha receptor, which on stimulation produce a decrease in cyclic AMP.
2.Beta receptor,which on stimulation produce rise in cyclic AMP and result in
inhibition of uterine contraction

14. Mechanism of Normal labour:
• The series of movements that occur on the head in the process of
adaptation during its journey through the pelvis is called
mechanism of labour .
Lie- longitudinal
Presentation -Cephalic
Presenting part-vertex
Denominator- occiput
Attitude- flexon
Engaging diameter- AP: Suboccipitobregmatic
Transverse- biparietal
Position- LOA

15. Stage of labor
• First stage: it start from onset of true labor and end with full dilation of
cervix. its average duration is 12 hour in Primigravida and 6 hour in
multigravida
• Second stage: its start from full dilation of cervix and end with expulsion
of fetus from birth canal. its has two phase:
Propulsive phase: start from full dilation up to the descent of the presenting part of
fetus to pelvic floor.
Expulsive phase: is from time mother has irresistible desire to bear down and push
until the baby is delivered.
Duration: 2 hour in Primigravida and ½ hour in multigravida.

16. Third stage
It begins after expulsion of the fetus and end with expulsion of
the placenta and membrane. 15 min. for Primigravida and
multipara
Main events in third stage of labor:
i) Placental seperation: central (Schultze) and marginal (Mathews
ducan )
ii) Expulsion of placenta

17. Fourth stage
• It is the stage of observation for at least 1 hour after expulsion
of the after births.
• Pulse , BP, behavior of uterus and abnormal vaginal bleeding is
to be watched.

18. TERMINOLOGY
1. LIE: The relation of the long axis of the fetus to that of
Mother uterus.
a) Longitudinal lie
b) Transverse lie
2. ATTITUDE: Posture of the fetus
a) Head flexed over the chest
b) Arms/hands flexed over the chest
c)Thighs/legs flexed over the abdomen

19. 3. PRESENTATION: is the part of fetus which lies over pelvic inlet and
occupies the lower pole of uterus.
4. POSITION :relation of an arbitrary chosen point of the fetal presenting part
to the right or left side of the maternal birth canal.

20. The cardinal movements in labour are:
1.Engagement
2.Descent
3.Flexion
4.Internal rotation
5.Crowning
6.Extension
7.Restitution
8.External rotation
9.Expulsion

22. • In good uterine contraction, adequate pelvis and average weight baby
• Longitudinal lie, cephalic presentation and LOA position
• ENGAGEMENT:
• The mechanism by which biparietal diameter-the greatest transeverse diameter
passes through the pelvic inlet.
• In multipara engagement occurs in the late first stage after rupture of membranes.
• In nullipara during the last few weeks of pregnancy.

23. • The fetal head tends to accommodate to the transverse axis of the pelvic inlet,
• Where the saggital suture while remaining parallel to that axis ,may not lie
exactly between the symphysis pubis and sacral promontory,
• It may either deflected anteriorly towards symphysis pubis or posteriorly towards
sacral promontory.
• Such deflection of head in relation to pelvis is called asynclitism

25. DESCENT:
• It is a continuous process.
• In nullipara engagement may take place before the onset of labor.
• In multiparas, descent usually begins with the engagement.
• descent is brought about by :
1.Pressure of the amniotic fluid
2.Direct pressure of the fundus upon the breech with contraction
3.Bearing down efforts of maternal abdominal muscles
4.Extension and straightening of the fetal body

26. FLEXION:
• It is achieved either due to the resistance offered by the unfoulding cervix,the
walls of the pelvis or by the pelvic floor.
• The chin is brought in to more intimate contact with the fetal thorax,
• And the shorter suboccipitobregmatic diameter is substituted for the longer
occipitofrontal diameter.

27. INTERNAL ROTATION:
• It is movement that turns the occiput gradually away from the transverse axis.
• the occiput rotates anteriorly toward the symphysis pubis.
• but less commonly, it may rotate posteriorly toward the hollow of the sacrum.
• Internal rotation is essential for completion of labor
• Two-third of women in labour,internal rotatation is completed by the time the head
reaches the pelvic floor.
• In One-fourth-internal rotation is completed shortly after the head reaches the
pelvic floor.
• In remaining internal rotation does
• Not take place.

28. • In nullipara, when the head fails to turn until reaching the pelvic floor,
• It typically rotates during the next one or two contractions.
• In nullipara, rotation occurs during the next three or five contractions.

29. CROWNING:
• Crowning is the maximum diameter of the head (biparietal diameter) stretches
the vulval outlet without any recession of the head even after the contraction is
over.

30. EXTENSION:
• Delivery of the head takes place by extension through “couple of force” theory.
• The driving force pushes the head in a downward direction,
• the pelvic floor offers a resistance in the upward and forward direction.
• The downward and upward forces neutralize and remaining forward thrust helping
in extension.
• The successive parts of the fetal head to be born through the stretched vulval
outlet are vertex, brow, face,mouth and chin.

31. RESTITUTION:
• Visible passive movement of the head due to untwisting of the neck sustained
during internal rotation.
• Movement occurs rotating the head through one-eighth of a circle in the direction
opposite to that of internal rotation

32. EXTERNAL ROTATION:
• It is the movement of rotation of the head visible externally due to internal rotation
of the shoulders
• The anterior shoulder rotates toward the symphysis pubis,
• And carries the head in a movement of external rotation through one-eighth of a
circle in the same direction as restitution.
• The shoulders now lie in the anteroposterior diameter.

33. EXPULSION:
• The shoulders are positioned in anteroposterior diameter of the outlet,
• Further descent takes place until the anterior shoulder escapes below the
symphysis pubis.
• By a movement of lateral flexion of the spine, the posterior shoulder sweeps over
the perineum.
• Rest of trunk expelled out by lateral flexion.