Mechanisms and Management
of Labor
Nancy Goodwine Wozniak, MD
Definition of Labor
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Labor is the physiologic process by which the fetus is
expelled from the uterus to the outside world
Could also be described as the transition from
“contractures” to “contractions”
Bottom line defination: Contractions with cervical
change. The diagnosis is a clinical one.
Gabbe: Obstetrics Normal and Problem pregnancies 4th edition
Full term pregnancy is 280 days (40 weeks)
or 36 completed weeks.
Post term pregnancy is beyond 42 weeks
SROM is seen in about 8% of patients
Labor Physiology
Labor is contractions with cervical change
The fetus is in control of the timing of labor
The factors responsible for initiating labor are not well-defined…likely an
autocrine and/or paracrine event.
We do know there is some endocrine maternal/fetal cross talk (eg horses and
donkeys indicate that fetal genotype is a factor—365 vs 340 days)
Gabbe: Obstetrics Normal and Problem pregnancies 4th edition
Labor Physiology
No matter what seems to initiate labor it involves regular uterine
contractions, mediated through ATP-dependent binding of myosin to
actin. Unlike vascular smooth muscle, myometrium has sparse
innervation, thus regulation of contractions is hormonal.
There is thought to be a parturition cascade. Ultimately, human labor
is a multifactorial physiologic process involving an integrated set of
changes that occur gradually over days to weeks. Changes include
prostaglandin synthesis and release within the uterus, an increase in
myometrial gap junction formation, and up-regulation of myometrial
oxytocin receptors. At some point labor begins with the activation
of the fetal-hypothalamic-pituitary adrenal axis in a way likely
common to all species.
Gabbe: Obstetrics Normal and Problem pregnancies 4th edition
Labor Physiology
The regulation of uterine activity can be divided into 4
physiologic phases
Phase 0: uterus is quiet due to progesterone, relaxin,
prostacyclin I2 (PGI2), parathyroid hormone
Phase 1: before term “activation” phase- uterus is more
responsive to estrogen and more receptors for oxytocin and
prostaglandins
Phase 2: uterus more stimulated because of increase in gap
junctions so that it can be stimulated by oxytocins and
prostaglandins (PGE2 and PGF2 alpha)
Phase 3: involution of the uterus (mediated by oxytocin)
Gabbe: Obstetrics Normal and Problem pregnancies 4th edition
Labor Mechanics
For a successful vaginal delivery, the fetus
must negotiate the maternal pelvis.
Three factors: the power, the passage, and
the passenger.
Labor Mechanics
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The passenger
Estimating fetal size: ultrasound, leopolds, what does
mom think?
How big is too big? Definition of macrosomia is
diabetics: 4500g non-diabetics: 5000g
Labor Mechanics
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Power
Assessing amplitude, duration, and intensity of ctx
internal IUPC vs external toco
What’s adequate contractions? (ultimately it is a clinical dx)
3-5 ctx in 10 min
7 ctx in 15 min
250 MVU’s – the average strength of ctx in mm Hg multiplied by
the number of contractions in 10 minutes. No real data support
an absolute number of ctx or MVU’s to be adequate…adequacy
is still a clinical determination.
If ctx are adequate either the cervix will dilate or the caput will
become worse.
Labor Mechanics
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The most precise way of determining uterine
contractions are adequate is with internal
monitoring by IUPC
External monitoring measures the change in
shape of the abdominal wall relative to
contractions thus is qualitative rather than
quantitative. Does allow for accurate correlation
between fetal heart rate and contraction pattern
.
Labor Mechanics
The Passenger
The passenger is the fetus. Fetal size can influence labor
Can be assessed by Leopold’s, US or both. ( Mom’s
opinion counts, too!)
ACOG definition of Macrosomia is defined as >4500 g
Labor Mechanics
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The passenger
Fetal lie: Fetal position relative to the maternal spine.
longitudinal, oblique, transverse
Presentation: refers to the fetal part that is above the pelvic inlet. (eg a fetus can have a logitudinal
lie but be breech or cephalic)
Attitude: refers to position of fetal head relative to the fetal spine
Position: referes to the relationship of a nominated site of the presenting part to a denomintating
location in the internal pelvis. Eg. Occiput/sacrum ROA, RSA
Station: a measure of descent of the presenting part.
Abnormalilty of any of these variables can influence whether or not to proceed with a vaginal delivery.
Fetal presentation: Fetal part directly over the pelvic inlet;
eg breech, cephalic, compound, funic
Labor Mechanics
 The
passenger
Malpresentation is any presentation that is
not cephalic with occiput leading. (about
5%) Multifetal pregnancies increase the
risk of malpresetnation
The cephalic presentation can be classified by
boney landmarks of the skull; eg occiput ,
mentum, brow
passenger
pasenger
A: Right occiput anterior (ROA); B: Left occiput anterior (LOA); C: Occiput
anterior (OA).
* Posterior fontanel. This is the smaller of the two fontanels and is at the
intersection of the three sutures: the sagittal suture and two lambdoid sutures.
** Anterior fontanel. This large fontanel is at the intersection of four sutures: the
sagittal, frontal, and two coronal sutures.
From UpToDate.com
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Occiput posterior
From UpToDate.com
Occiput transverse
From UpToDate.com
Labor Mechanics
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The passenger
Station: measure of descent of the presenting part
through the birth canal relative to ischial spines
this is the relationship between the leading bony part of fetal
presenting part ( skull bone NOT scalp) and the maternal ischial spines.
Must take into account molding and caput succedaneum (not doing so
is a common error)
Often described as -3 to + 3
Newer scale is -5 to +5
Nucleus medical art.
Nucleusinc.com
Labor Mechanics
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The Passage
The passage consists of the bony pelvis (sacrum, ilium, ischium, pubis)
and the resistance provided by the soft tissues.
Bony pelvis is divided into the greater (false) and lesser(true) pelvis by
the pelvic brim which is demarcated by the sacral promontory.
The diagonal conjugate is the distance from the sacral promontory to
the inferior margin of the symphysis pubis as assessed on
examination ( see next slide )
Clinical pelvimetry is the only way to assess the dimensions of the
pelvis in labor.
To figure out the true conjugate, measure the diagonal
conjugate and subtract 1.5 – 2cm. The limiting factor is
the interspinous diameter.
Bony pelvis—most favorable is gynecoid and antropoid
From UpToDate.com
A little bit about cervical ripening…
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When induction is attempted against an unripe cervix the likelihood
of succcess is reduced.
Bishops score: dilatation, effacement, position, consistency, station.
Total score is up to 13.
Bishop’s = 8 chances of successful induction are the same as
spontaneous labor
Bishop’s = 6 “favorable cervix”
(A) Cervix is uneffaced and minimally dilated. (B) Cervix is almost completely
effaced and dilated.
From UpToDate.com
Methods of cervical ripening
Non Pharmacologic methods:
membrane stripping – digital separation of chorionic and amniotic membranes from
the cervix. Releases endogenous prostaglandins from the decidua and adjacent
membranes.
May also cause “Ferguson reflex” stimulating release of oxytocin from the pituitary.
Foley bulb
Amniotomy – needs favorable cervix, but if cervix is favorable amniotomy by itself
can get labor started (better still when combined with Pitocin)
risks:
cord prolapse, prolonged ROM, fetal injury, rupture
of vasa previa with fetal hemorrhage, fetal malposition and
asynclitism
Benefits: FSE placement, can determine if MSF or blood, high success
in inducing labor
Methods of cervical ripening
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Pharmacologic methods
Dinoprostone (Prepadil and Cervadil) PGE2 , oxytocin,
misoprostyl (cytotec) PGE1
The uterus has precursors of the prostaglandin of the 2
series.
PGE2 : important for cervical maturation
PGF 2 alpha. : causes myometrial
contractions
Stages of Labor
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First stage: Onset of labor to full dilatation
latent phase- onset of labor until cervix starts to make
change.
active phase-greater rate of cervical change
1.2 cm/h for nulliparous
1.5 cm/h for multiparous
Second stage: full dilation to delivery
Length of Pushing: nullip: 2h without epidural, 3 h
with epidural
multip: 1 h without epidural,
2 h with epidural
Third stage: delivery of placenta-can take up to 30 minutes
Cardinal movements of labor
Engagement: passage of the widest diameter of the presenting part to
a level below the plane of the pelvic inlet. In cephalic fetus, the
largest diameter is the biparietal diameter (9.5 cm); in a breech
fetus the widest diameter is the bitrochanteric diameter. The
presenting part is engaged if you can feel presenting part both
abdominally and vaginally.
Descent: downward passage of the presenting part
Flexion: occurs passively d/t boney maternal pelvis
Internal rotation: refers to rotation of presenting part from its original
position (usually transverse) to the AP position
Extension: Occurs once the fetus has descended to the introitus
External rotation (aka restitution) return of the fetal head to the correct
anatomic position in relation to the fetal torso.
Expulsion: delivery of the rest of the fetus.
Management of Normal Labor and Delivery
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All women need adequate surveillance throughout labor
and delivery.
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Okay to let women walk (doesn’t shorten course of labor, the
need for augmentation, the use of analgesia, or the rate of C/S)
Record FHT’s q 30 minutes (minimum)
During second stage, FHT’s should be recorded q 15 and after
each contraction
Augmentation of Labor at Term
Abnormalities of the first stage of labor may be either protraction or arrest
disorders and can occur during active or latent phases of labor.
--Administer Pitocin as long as no malpresentation.
Goal is ctx q 2-3 min lasting 60-90 seconds. Resting tone should be
10-15 mm Hg if IUPC is used.
--Takes 30 to 40 minutes to see full effect of Pitocin dose
--A slow rate of pitocin increase is as effective as a fast rate.
--Whether to add pitocin to a patient who is already adequately contracting
is controversial, but 80% of patients will respond to pitocin
Augmentation of Labor at Term
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Advantages:
Oxytocin is cheap, and well known to us
Short t1/2
Complications:
uterine hyperstimulation (tachysystole)
increased uterine tone (hypertonia)
water intoxication (at doses of 30-40 miu since it’s a vasopressin
analogue)
hypotension (usually if pitocin is given as a bolus)
uterine rupture (associated with “excessive oxytocin”)
Abnormal patterns of labor
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“Latent phase arrest”—means labor never began
“prolonged latent phase”—greater than 20h in nullip
---greater than 14h in multip
Prolongation of latent phase is variable; doesn’t mean the fetus will
have a bad outcome or that the patient needs a c/s. Can be
managed expectantly (presuming mom and baby other wise look
good)
Can administer analgesics (eg morphine 15-20 mg for
therapeutic rest)
Augmentation (Pitocin)
Defer amniotomy!
Pitocin regimen
Regimen
Starting dose
Incremental
increase
Dosage change
interval in
minutes
Low dose
0.5-1.0 mu
1 mu
30-40 minutes
Alternative low
dose
1-2 mu
2
15 minutes
High dose
6 mu
6 mu
Max 40 mu
15 minutes
Alternative high
dose
4 mu
4 mu
Max 32 mu
15 minutes
Abnormal patterns of labor
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Abnormalities of second stage
“Failure to Progress” “Arrest of dilatation”
generally patient is falling off Friedman’s curve, or
no cervical change in 2 hours
Consider augmentation, placement of IUPC
Abnormal patterns of labor
“Protraction of descent”
Descent of < 1 cm/h in nullips
Descent of < 2 cm/h in multips
Deliveries complicated by prolonged second stage put the fetus at
risk of acidosis, thus, ACOG recommends intervention after 2 h
without epidural, 3 h with epidural.
In reality, can consider expectant management if mother and fetus
are otherwise reassuring, descent is progressive, and delivery is
imminent.
Abnormal patterns of labor
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“Arrest of Descent”
This requires an assessment of contractions,
maternal fetal well being, and CPD
Re-evaluate clinical pelvimetry, fetal station, caput.
The decision to proceed with assisted vaginal delivery or
C/S should be individualized
Do you really want to do that episiotomy??
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Episiotomy– the incision in the perineal body during the second
stage of labor.
Indicated in 1) cases of arrested or protracted descent
2) expedite delivery in NRFHT’s
Median: performed when the fetal head is on the perineum.
Associated with occasional extensions to 3rd or 4th degree
Mediolateral: 45 degree angle from the hymenal ring. Does not
increase risk of 3rd or 4th degree extension. Procedure of choice in
patients with inflammatory bowel disease. More pain post partum.
Uptodate.com
Episiotomy
Fewer episiotomies are being performed…most
repairs after a vaginal delivery are a result of
tears.
Episiotomies (and lacerations) are graded on a
scale of 1 to 4
Episiotomy/Lacerations
1st degree lacerations: involve the forchette, perineal skin,
and vaginal mucosa
2nd degree lacerations: above plus extend to the fascia and
muscles of the perineal body but not to the anal
sphincter
3rd degree lacerations: skin, mucosa, perineal body and
anal sphincter
4th degree: exposed lumen of the rectum
Vaccuum Deliveries
Vaccuum Deliveries
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Vaccuums have been around since 1953
By 1970’s popular in Northern Europe
Didn’t exceed number of forceps
deliveries in the U.S. until 1992
Fetal contraindications to a Vaccuum
1) < 34 weeks
increases risk of intraventricular hemorhage
2) Fetal bleeding diathesis e.g., ITP,
hemophilia
3) Multiple FSE attempts
4) CPD
Vaccuum types
Take a look at what we have!
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Optimum type…who knows
Can use any of them if no
contraindication
In general…soft cups, more likely to fail but less
fetal scalp injury; rigid cups probably better for
OP
A Vaccuum does not require less clinical
knowledge than forceps!
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Must know fetal position, station, and take
into account molding
Must know contraindications
Placement of cup now becomes flexion point.
Unlike forceps which can be used to correct asynclitism, a
vaccuum will impede delivery if cup not placed over
flexion point.
Check list prior to instrumental delivery
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Empty bladder
Dorsal lithotomy position
Adequate anesthesia ( a MUST for
forceps!)
Fetal position, station, EFW
Putting on the Vac
Determine flexion point:
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basically flexion point is
the point where pulling is going to best allow flexion at
the neck keeping the fetus OA.
Midline, over sagital suture, 6 cm from Anterior
fontanelle, 3 cm from posterior fontanelle.
Anterior fontanelle has to be your reference point.
360 degree inspection
Green zone to 450
The instrumental delivery itself
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Pull along pelvic curve (down, then up)
Let handle passively turn as fetus rotates
with delivery
Descent should occur with each pull
No routine episiotomy
How long is too long?
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No one knows maximal amount of time
and maximal amount of pop-offs that is
acceptable
Ideally less than 15 minutes, certainly less than 30
Usually less than 3 pop-offs, less than 5 pulls
Documentation
Indications
Were prerequisites met
(full dilatation, empty
bladder, no contraindications, gest. Age, station (+2/3 or
+2/5??)
Fetal status (station, position, FHT’s
Verbal consent
Detailed description of procedure
Type of vaccuum, total time, reduced between
contractions, # pulls, # ctx, # pop-offs, progress with
each pull, epis or not
Reasons instrumental deliveries fail
CPD
Bad technique
(eg pulling without contractions,
upward pull before crowning: deflexed, paramedian
application
Large Caput
Remember…No one thanks you for a vaginal
delivery unless its perfect.
Shoulder dystocia
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www.shoulderdystociaattorney.com
If the anterior and posterior shoulders descend together instead of sequentially, the
anterior shoulder can become impacted behind the symphysis pubis (or the posterior
shoulders on the sacral promontory)
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If descent of the fetal head continues
while the shoulders remain impacted,
stretching of the nerves of the brachial
plexus can occur.
Most brachial plexus injuries resolve on
their own, but permanent injury is a often
a medicolegal issue.
Risks for shoulder dystocia
maternal obesity, diabetes, post dates,
macrosomic infant, operative delivery
Other risks associated with shoulder
dystocia: fetal hypoxia and neurologic
injury; fractured clavical or humerus, fetal
death.
Management of Shoulder dystocia
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Call for help!
Suprapubic pressure
McRoberts Maneuver
Episiotomy
Woods screw/ Ruben’s manuevers
Deliver posterior arm
Fracture clavicles
Zavenelli maneuver
Mom should not push during maneuvers!!
Henry Lerner, MD
Graphics Susan Seif, medical graphics
After difficult delivery…
Careful documentation
Explain to patient the events, explanation of
problem, steps taken to correct the problem,
and what the anticipated sequelae are
Fetal Monitoring
The following examples of fetal monitoring strips are from…
Interpretation of the Electronic Fetal Heart Rate
During Labor
AMIR SWEHA, M.D., and TREVOR W. HACKER, M.D.
Mercy Healthcare Sacramento
Sacramento, California
NUOVO, M.D.
University of California, Davis, School of Medicine
Davis, California
Used with permission from The American Family Physician
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Figure 1 Interpreting fetal monitiring Strips; American Academy of Family
Physicians May, 1999
Figure 1
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Interpretation of Electronic Fetal Heart rate During Labor
American Family Physician, May 1999
Figure 1
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Reassuring pattern. Baseline fetal heart
rate is 130 to 140 beats per minute
(bpm), preserved beat-to-beat and longterm variability. Accelerations last for 15
or more seconds above baseline and peak
at 15 or more bpm. (Small square=10
seconds; large square=one minute
Figure 2
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Interpretation of Electronic Fetal Heart rate During Labor
American Family Physician, May 1999
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FIGURE 2. Saltatory pattern with wide
variability. The oscillations of the fetal
heart rate above and below the baseline
exceed 25 bpm.
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Fetal tachycardia with possible onset of
decreased variability (right) during the
second stage of labor. Fetal heart rate is
170 to 180 bpm. Mild variable
decelerations are present.
Figure 3
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Interpretation of Electronic Fetal Heart rate During Labor
American Family Physician, May 1999
Figure 3
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Fetal tachycardia that is due to fetal
tachyarrhythmia associated with
congenital anomalies, in this case,
ventricular septal defect. Fetal heart rate
is 180 bpm. Notice the "spike" pattern of
the fetal heart rate.
Figure 4
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Interpretation of Electronic Fetal Heart rate During Labor
American Family Physician, May 1999
Figure 4
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Early deceleration in a patient with an
unremarkable course of labor. Notice that
the onset and the return of the
deceleration coincide with the start and
the end of the contraction, giving the
characteristic mirror image.
Figure 5
Figure 5
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Nonreassuring pattern of late
decelerations with preserved beat-to-beat
variability. Note the onset at the peak of
the uterine contractions and the return to
baseline after the contraction has ended.
The second uterine contraction is
associated with a shallow and subtle late
deceleration
Figure 6
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FIGURE 6. Nonreassuring pattern of late decelerations
with preserved beat-to-beat variability. Note the onset at
the peak of the uterine contractions and the return to
baseline after the contraction has ended. The second
uterine contraction is associated with a shallow and
subtle late deceleration
Figure 7
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Figure 7
. Late deceleration with loss of variability. This is an ominous pattern, and immediate delivery is indicated
Figure 8
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FIGURE 8. Variable deceleration with pre- and post-accelerations ("shoulders").
Fetal heart rate is 150 to 160 beats per minute,
and beat-to-beat variability is preserved.
Figure 9
Figure 9
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Severe variable deceleration with
overshoot. However, variability is
preserved.
Figure 10
Figure 10
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FIGURE 10. Late deceleration related to bigeminal contractions.
Beat-to-beat variability is preserved. Note the prolonged contraction
pattern with elevated uterine tone between the peaks of the
contractions, causing hyperstimulation and uteroplacental
insufficiency. Management should include treatment of the uterine
hyperstimulation. This deceleration pattern also may be interpreted
as a variable deceleration with late return to the baseline based on
the early onset of the deceleration in relation to the uterine
contraction, the presence of an acceleration before the deceleration
(the "shoulder") and the relatively sharp descent of the
deceleration. However, late decelerations and variable decelerations
with late return have the same clinical significance and represent
nonreassuring patterns. This tracing probably represents cord
compression and uteroplacental insufficiency.
Figure 11
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FIGURE 11. (A) Pseudosinusoidal pattern. Note the
decreased regularity and the preserved beat-to-beat
variability, compared with a true sinusoidal pattern (B).
When all else fails….
Enough already!!!