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Mechanisms and Management of Labor. Nancy Goodwine Wozniak, MD. Definition of Labor. 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 ”

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mechanisms and management of labor

Mechanisms and Management of Labor

Nancy Goodwine Wozniak, MD

definition of labor
Definition of Labor
  • 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 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 physiology1
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 physiology2
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
Labor Mechanics

For a successful vaginal delivery, the fetus must negotiate the maternal pelvis.

Three factors: the power, the passage, and the passenger.

labor mechanics1
Labor Mechanics
  • 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 mechanics2
Labor Mechanics
  • 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 mechanics3
Labor Mechanics
  • 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 mechanics4
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 mechanics5
Labor Mechanics
  • 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
Fetal presentation: Fetal part directly over the pelvic inlet; eg breech, cephalic, compound, funic
labor mechanics6
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
The cephalic presentation can be classified by boney landmarks of the skull; eg occiput , mentum, brow




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

Occiput posterior


occiput transverse
Occiput transverse


labor mechanics7
Labor Mechanics
  • 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

labor mechanics8
Labor Mechanics
  • 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.
a little bit about cervical ripening
A little bit about cervical ripening…
  • 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

methods of cervical ripening
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 ripening1
Methods of cervical ripening
  • 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
Stages of Labor
  • 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
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
Management of Normal Labor and Delivery
  • All women need adequate surveillance throughout labor and delivery.
    • 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
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 term1
Augmentation of Labor at Term
  • Advantages:

Oxytocin is cheap, and well known to us

Short t1/2


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
Abnormal patterns of labor
  • “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!

abnormal patterns of labor1
Abnormal patterns of labor
  • 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 labor2
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 labor3
Abnormal patterns of labor
  • “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
Do you really want to do that episiotomy??
  • 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.


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 deliveries1
Vaccuum Deliveries
  • 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
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
Vaccuum types

Take a look at what we have!

  • 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
A Vaccuum does not require less clinical knowledge than forceps!
  • 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
Check list prior to instrumental delivery
  • Empty bladder
  • Dorsal lithotomy position
  • Adequate anesthesia ( a MUST for forceps!)
  • Fetal position, station, EFW
putting on the vac
Putting on the Vac

Determine flexion point: 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
The instrumental delivery itself
  • 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
How long is too long?
  • 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


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
Reasons instrumental deliveries fail


Bad technique(eg pulling without contractions, upward pull before crowning: deflexed, paramedian application

Large Caput


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)

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
Management of Shoulder dystocia
  • 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!!
after difficult delivery
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
Fetal Monitoring

The following examples of fetal monitoring strips are from…

Interpretation of the Electronic Fetal Heart Rate During Labor


Mercy Healthcare Sacramento

Sacramento, California


University of California, Davis, School of Medicine

Davis, California

Used with permission from The American Family Physician

Figure 1 Interpreting fetal monitiring Strips; American Academy of Family Physicians May, 1999
figure 1
Figure 1
  • Interpretation of Electronic Fetal Heart rate During Labor

American Family Physician, May 1999

figure 11
Figure 1
  • Reassuring pattern. Baseline fetal heart rate is 130 to 140 beats per minute (bpm), preserved beat-to-beat and long-term 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
Figure 2
  • Interpretation of Electronic Fetal Heart rate During Labor

American Family Physician, May 1999

FIGURE 2. Saltatory pattern with wide variability. The oscillations of the fetal heart rate above and below the baseline exceed 25 bpm.
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
Figure 3
  • Interpretation of Electronic Fetal Heart rate During Labor

American Family Physician, May 1999

figure 31
Figure 3
  • 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
Figure 4
  • Interpretation of Electronic Fetal Heart rate During Labor

American Family Physician, May 1999

figure 41
Figure 4
  • 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 51
Figure 5
  • 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. 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

. Late deceleration with loss of variability. This is an ominous pattern, and immediate delivery is indicated


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 91
Figure 9
  • Severe variable deceleration with overshoot. However, variability is preserved.
figure 101
Figure 10
  • 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 111
Figure 11
  • FIGURE 11.(A) Pseudosinusoidal pattern. Note the decreased regularity and the preserved beat-to-beat variability, compared with a true sinusoidal pattern (B).