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
5.
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
9.
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
12.
13.
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
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.
