This document discusses the physiological changes and management of normal labor. It covers topics such as: - The initiation of labor and the progression of painful and regular contractions leading to cervical dilatation. - The physiology behind labor contractions including hormonal control, electrical activity in the myometrium, gap junctions, prostaglandins, and changes in contraction pattern throughout pregnancy and labor. - The mechanics of labor which involves the interaction between uterine activity (contraction strength), the fetus (size, position, etc.), and the maternal pelvis (shape and dimensions). - Methods of assessing uterine contractions and the variables that can impact the course of labor such as fetal size, position, and station

1. LABOR:
PHYSEOLOGICAL CHANGES
MANAGEMNT OF NORMAL LABOR

2. Initiation of labor
 Cervical changes initiated earlier
 Labor: Myometrial contractions leading to
cervical dilatation
 Continues painful and regular contractions
progressively increasing in intensity and frequency
and leading to
 Progressive cervical dilatation
 Since the process (physiology) of labor
involves continues myometrial and cervical
preparation through out pregnancy, onset of
labor remains elusive.

3. Physiology of labor:
contractions
 Uterus contracts through out pregnancy:
 Irregular in timing (frequency)
 Irregular in intensity
 Discoordinate in distribution
 Mostly painless No cervical change
 Contractions in late third trimester (false labor):
 more frequent but on & off
 greater intensity, and more consistent in intensity
 more coordinated
 Usually some pain (with contractions) effacement, dilatation, lower
segment formation
 Contraction in labor:
 Regular, frequent, intensity increasing
 Well coordinated with impulse flow from cornual area to the lower uterine
segment faster rate of effacement and descent of the fetus

4. Contractions and retraction
 Actin-myosin interaction is regulated by CA++
 Rise in [Ca++ ]IC
++ triggers muscle contraction
 Many factors regulate CA++ flow into
myometrial cells and from intracellular stores
 Myocyte stretching also initiates contraction

6. Labor contraction:
Electrical activity & contractions
 Uterine contractions are effected through electrical
activities of myometrial cells
 During pregnancy, the pattern of electrical activity in the
myometrium changes from irregular spikes to regular
activity
 As labor progresses, electrical activity becomes more
organized and increases in amplitude and duration.
 With coordination of electrical activities in labor, uterine
contractions become more coordinated and stronger.

7. Labor contraction:
Gap junctions (GJ) & electrical activity
 GJ are intracellular channels
 When GJ open, they facilitate electrical and metabolic
communication between myometrial cells
 GJ number & permeability increases during active labor
(and late pregnancy)
 Coordination of contractions depends on GJ.
 Progesterone suppresses the number and permeability
of GJ, while estrogen is associated with the reverse.
 Immediately after delivery, GJ are degraded

8. Labor contraction and PG
 Increased in labor
 PG mediated contraction is achieved by increased [Ca++ ]IC
 increasing [Ca++ ]IC by influx across the cell
membrane and release from IC stores
 enhancing formation of GJ
 Effect depends on type of PG
 Contraction: PGF2α, Thromboxane, PG E3
 Inhibition of contraction: PG D, PG E2, PG I

10. Initiation of labor and hormonal control
 Progesterone
 With the onset of labor, the proportion of progesterone receptors types change in
a way that lead to progesterone withdrawal effect that ameliorate its relaxation
effect on the myocytes
 Estrogen
 With progesterone functional withdrwal, estrogen becomes effective and
formation of its receptors increased
 GJ formation
 Oxytocin receptors increament
 Oxytocin
 Receptors increase from early to late pregnancy and labor
 Oxytocin receptor concentration is increased by estrogen and
decreased by progesterone
 Stimulation leads to [Ca++ ]IC increase

11. Corticotropin-releasing hormone (CTH)
 Associated with the development of the placenta;
especially expression of the gene for CTH
 Increasing exponentially as pregnancy advances,
peaking at the time of delivery – the rate of increase is
an important indicator of it effect
 At end of pregnancy, CRH-binding protein (CRHBP) fall,
decreasing bioavailability of CRH leading to increase in cortisol
production (which includes a positive feed-forward system)
 At term, the CRH receptors change from one that leads to
relaxation of the myometrial cells to receptors leading to
contraction

14. Labor contraction and hormonal control
 Endothelin:
 Receptors found in chorion, endometrium, and myometrium
 Receptors increase during labor
 Enhances uterine contraction by:
 Increasing [Ca++ ]IC influx
 Stimulating PG production
 Epidermal growth factor (EGF)
 Receptors present in endometrium and myometrium
 Induced by estrogen
 Increases [Ca++ ]IC

15. Myometrial contraction pattern
during pregnancy, labor & delivery
Uterine
contractility
Quiescence
Inhibitors
Activation
Uterotrophins
Stimulation
Uterotonins
Involution
Progesterone
PGi2
Relaxin
NO etc
Estrogen
GJ
Ion
channels
PG
Oxytocin
Oxytocin
Phase 0 Phase 1 Phase 2 Phase 3

16. MECHANICS OF LABOR
 The ability of the fetus to successfully
negotiate the pelvis during labor and delivery
depends on the complex interaction of three
variables:
 uterine activity (Powers)
 Fetus (Passenger)
 maternal pelvis (Passage).

17. Uterine Activity:
Power generated by uterine musculature
 Uterine activity characterized by:
 frequency,
 amplitude (intensity),
 duration of contractions
 Adequate labor:
 3-5 contractions /10 minutes in 95% of spontaneous labor
 Every 2 - 5 minutes in early labor to 2 - 3 minutes in late active
labor and during the second stage.

18. Effect of adequate uterine contractions
 Effacement, dilatation, descente: vaginal
delivery
OR
 Increasing caput succedaneum, molding, slow
effacement, dilatation:
 CPD: absolute or relative (malposition, abnormal attitude)

19. Methods of
uterine activity assessment
 Simple observation,
 Manual palpation,
 External tocodynamometry:
 Contractions Abdominal shape change Graphic uterine activity
 Correlates FHR with uterine activity BUT NOT contraction intensity or
basal intrauterine tone.
 Internal tocodynamometry via internal uterine pressure catheter
 Most precise method
 Performed with indication
 Risks: uterine perforation, placental disruption, intrauterine infection
(HIV)

20. Contraction
measurement
 Montevideo unit: most common objective measure
 Measures average frequency and amplitude above basal tone
 Average strength of contractions in mmHg multiplied by number
of contractions per 10 minutes
 Adequate labor in the active phase of labor: 200 to 250 MU
 Abnormal uterine activity
 Tachysystole: more than 5 contractions in 10 minutes for at least
20 minutes
 Hyperstimulation: tachysytole accompanied by abnormal FHR

21. The Fetus (Passenger):
Fetal variables influence course of labor & delivery
 Fetal size: abdominal palpation or ultrasound
 Macrosomia: actual birth weight greater than 4,000 g
 Increased likelihood of failed trial of labor
 Lie : longitudinal axis of the fetus relative to the
longitudinal axis of the uterus
 Longitudinal, transverse, or oblique

22. The Fetus (Passenger):
Fetal variables influence course of labor &
delivery
 Malpresentation: any presentation other than vertex
 5% of all term labors
 Attitude: position of head with fetal spine
 Flexion facilitates engagement
 Chin optimally flexed onto the chest: suboccipitobregmatic diameter (9.5
cm)
 Deflexed (extended) head: brow and face

23. The Fetus (Passenger):
Fetal variables influence course of labor & delivery
 Position: relationship of the fetal presenting part to the maternal
pelvis
 Malposition refers to any position in labor that is not ROA, OA, or
LOA
 Station: measure of descent of the bony presenting part of the
fetus through the birth canal
 Classification (-5 to +5) based on a quantitative measure in
centimeters of the distance of the leading bony edge from the
ischial spines
 Descent using remaining part of head above pubis S.

24. The Maternal Pelvis (Passage)
 Consists:
 bony pelvis (composed of the sacrum, ilium,
ischium, and pubis)
 soft tissues

25. Parts of the bony pelvis
 Bony pelvis divided by the pelvic brim* into:
 false (greater)
 true (lesser) pelvis
 Classification of birth canal:
 pelvic inlet
 Midpelvis
 pelvic outlet
* Pelvic brim is demarcated by:
sacral promontory
anterior ala of sacrum
arcuate line of ilium
pectineal line of pubis
pubic crest

26. Shapes of bony pelvis
Four broad categories: Gynecoid, anthropoid, android, platypelloid
 Gynecoid pelvis:
 Classic female shape
 Oval-shaped inlet,
 Diverging midpelvic sidewalls,
 Far-spaced ischial spines
 Anthropoid pelvis:
 Exaggerated oval shape to the inlet
 Largest diameter being anteroposterior
 Limited anterior capacity to the pelvis
 More often associated with occiput posterior position

27. Shape of bony pelvis
Four broad categories: Gynecoid, anthropoid, android, platypelloid
 Android pelvis:
 Male in pattern
 Heart-shaped inlet
 Prominent sacral promontory
 Prominent ischial spines
 Shallow sacrum
 Converging midpelvic sidewalls
 Increased risk of CPD.
 platypelloid pelvis:
 Broad, and flat pelvis
 Exaggerated oval-shaped inlet
 Largest diameter being transverse diameter
 Theoretically predisposing to transverse arrest

28. Clinical pelvimetry
Assessed > 36 and in labor
 Inlet of true pelvis
 Transverse diameter: largest (>12.0 cm)
 True conjugate (obstetric conjugate): sacral
promontory to superior aspect of symphysis pubis
 TC= Diagonal C * - 1.5 (to 2.0) cm
 Normally ≈ 10 to 11 cm
Diagonal conjugate:
• Sacral promontory to inferior margin of the symphysis pubis

29. Clinical pelvimetry
 Midpelvis
 Limiting diameter is the interspinous diameter
 Normally: > 10 cm
 Pelvic outlet
 Rare clinical significance
 Anteroposterior diameter: coccyx to symphysis pubis≈13 cm
 Transverse diameter: inter-tuberose (ischial) ≈ 8 cm

30. Clinical pelvimetry
 Favorable pelvic shape for vaginal delivery:
 Favorable: gynecoid, anthropoid
 Less favorable: android, platypelloid
 Many pelvis of women fall into intermediate categories
 Although the assessment of fetal size along with pelvic shape and
capacity (x-ray) is still of clinical utility, it is a very inexact science
 An adequate trial of labor is the only definitive method to
determine whether a given fetus will be able to safely negotiate a
given pelvis
Past obstetric history best screening test

31. Soft tissue resistance
 Pelvic soft tissues may provide resistance in both the
first and second stages of labor
 In the first stage, resistance is offered primarily by the
cervix; whereas in the second stage, it is by the muscles
of the pelvic floor
 In the second stage of labor, the resistance of the pelvic
musculature is believed to play an important role in the
rotation and movement of the presenting part through
the pelvis.

32. Mechanisms of labor:
Cardinal movements
 Cardinal movements: changes in position of fetal head during its passage
through the birth canal.
 Due to asymmetry of the shape of both the fetal head and the maternal
bony pelvis, rotations are required for the fetus to successfully negotiate the
birth canal.
 Although labor and birth is a continuous process, seven discrete cardinal
movements of the fetus are described:
 engagement,
 descent,
 flexion,
 internal rotation,
 extension,
 external rotation or restitution,
 expulsion.

33. Engagement
 Passage of the widest diameter of the presenting part to a level below
the plane of the pelvic inlet
 Cephalic presentation with well-flexed head: biparietal diameter (9.5 cm).
 Breech: bitrochanteric diameter
 Owing to the angle of inclination between the maternal lumbar spine
and pelvic inlet, the fetal head engages in an asynclitic fashion.
Leading parietal eminence descends and is first to engage the pelvic
floor.
 Presenting part at 0 station (ischial spines)
 The pelvic inlet is sufficiently large to allow descent
 Timing of engagement
 Nullipara: usually by 36 weeks' gestation.
 Multipara: can occur later in gestation or even during the course of labor
 African: late in first stage of labor

34. Descent
 Downward passage of the presenting part
through the pelvis.
 Greatest rates of descent: deceleration phase
and second stage

35. Flexion
 Passively as the head descends owing due to:
 Shape of the bony pelvis
 Resistance offered by the soft tissues of the pelvic floor
 Flexion to some degree in most fetuses starts before
labor, and complete flexion usually occurs during the
course of labor
 Complete flexion presents the smallest diameter of the
fetal head (the suboccipitobregmatic diameter) for
optimal passage through the pelvis.

36. Internal Rotation
 Rotation of the presenting part from its original position as it enters
the pelvic inlet (usually OT) to the anteroposterior position as it
passes through the pelvis.
 As flexion, internal rotation is a passive movement resulting from
the shape of the pelvis and the pelvic floor musculature that forms a
V-shaped hammock that diverges anteriorly.
 As the head descends, the occiput rotates towards the symphysis
pubis (or, less commonly, towards the hollow of the sacrum),
thereby allowing the widest portion of the fetus to negotiate the
pelvis at its widest dimension.

37. Extension
 Extension occurs at the introitus.
 Base of the occiput into contact with the inferior margin
at the symphysis pubis at which point the birth canal
curves upward
 Head is delivered by extension and rotates around the
symphysis pubis by the downward force exerted on the
fetus by the uterine contractions along with the upward
forces exerted by the muscles of the pelvic floor.

38. External Rotation (restitution)
 Return of the fetal head to the correct
anatomic position in relation to the fetal torso
 Passive movement resulting from a release of
the forces exerted on the fetal head by the
maternal bony pelvis and its musculature and
mediated by the basal tone of the fetal
musculature

39. Expulsion
 Delivery of the rest of the fetus
 After delivery of the head and external rotation, further
descent brings the anterior shoulder to the level of the
symphysis pubis.
 The anterior shoulder is delivered with rotation of the
shoulder under the symphysis pubis.
 After the shoulder, the rest of the body is usually
delivered without difficulty.