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Intrapartum Physiology


Intrapartum Physiology How the baby enters this world True vs. False Labor (p. 541, Olds) True False Contractions Cervix Fetus Stages of Labor Stage 1 (closed ... – PowerPoint PPT presentation

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Title: Intrapartum Physiology

Intrapartum Physiology
  • How the baby enters this world

The 5 Ps of Labor
  • Passage
  • Birth canal
  • Passenger
  • Fetus
  • Powers
  • Forces of labor
  • Position
  • Fetal engagement and position
  • Psyche
  • Maternal behaviors

  • Passage (birth canal) consists of the pelvic
    inlet, midpelvis, and outlet
  • The size and type of the passageway affects the
    labor and delivery process and outcome

  • Four types of Passageway
  • Gynecoid
  • Adequate pelvic outlet
  • Round, slightly oval, 50 of women
  • Most favorable for delivery ?
  • Android
  • Normal male pelvis
  • Heart shaped, 20 of women
  • Not favorable for delivery
  • Higher risk of c-section, arrest of labor, forcep
  • Anthropoid
  • 25 of women, narrow arch
  • Favorable for labor
  • Platypoid
  • Flat pelvis, 5 of women
  • Favorability for labor ? due to outlet capacity

Broad upper hip
Strongest bone
Passage Inominate bones
Site of attachment of ligaments and muscles,
shortest diameter of pelvic cavity, serve as
reference point during labor evaluation of
descent of the fetal head into the birth canal
Supports the weight of the enlarged uterus
and directs the presenting fetal part into the
true pelvis
Represents the bony limits of the birth
canal Consists of pelvic inlet, midpelvis and
pelvic outlet
Where fetus enters true pelvis The size and
shape of the pelvic inlet are measured manually
during a pelvic exam by assessing the diagonal
conjugate. The diagonal conjugate measures the
diameter of the pelvic passage.
Pelvic cavity
Size of outlet is determined by the intertuberous
diameter Diameter may be increased by 1.5 to 2 cm
in squatting, sitting or dorsal lithotomy
position Hormones during pregnancy causes
relaxation of joints in the pelvis for an easier
passage of fetus
Diagonal conjugate
  • Distance from sacral promontory to symphysis
  • Approximate length of fingers introitus to sacrum
  • Adequate diagonal conjugate gt 11.5cm

Intertuberous Diameter
  • Distance between Ischial tuberosities
  • Approximately width of fist
  • Adequate intertuberous diameter gt 10 cm

Passenger Fetal Head Diameters are measured b/t
various landmarks on the skull
Extends from under surface of occiput to center
of ant. fontanel
Distance b/t bipatal protuberances, largest
transverse diameter of baby
Root of nose to occipital prominence
Chin to occiput
Passenger Bones of the Head
  • Fetal skull consists of three major parts, face,
    cranium, and vault
  • Vault consists of 2 fontal bones, 2 parietal and
    the occipital bones (these are not fused)
  • Overlapping of these bones to allow for delivery
    is called molding
  • Sutures are membranous joints that unite the
    cranial bones, sutures allow for molding of the
    fetal head and in identifying position of fetal
    head in the birth canal
  • Frontal suture b/t frontal bones
  • Saggital suture b/t parietal bones divides the
    skull into L and R halves, connects 2 fontanelles
  • Coronal suture b/t frontal and parietal
  • Lambdoidal suture located b/t parietal and

  • Attitude
  • The relation of the fetal body parts to one
  • Fetal posture is generally flexed
  • Head is flexed so that chin is on the chest and
    arms crossed over chest with legs flexed at the
    knees with thighs on abdomen
  • Lie
  • Relationship of the spinal column (long axis) of
    fetus to the spinal column (long cephalocaudal
    axis) of mother
  • Longitudinal or transverse
  • Presentation
  • Determined by fetal lie
  • Refers to the body part of the fetus that enters
    the pelvis first
  • Presenting part portion of the fetus felt through
    the cervix determines presentation
  • Vertex (cephalic most common), breech, face,

  • Position
  • Assesses relationship of pelvis and presenting
  • Engagement of the presenting part occurs when the
    largest diameter of the presenting part passes
    through the pelvic inlet
  • Determined by VE, occurs 2 weeks before term
  • Multiparas may experience several weeks before
  • Floating (bollotable) freely movable above pelvic
  • Dipping beginning to descend to inlet before
  • Station
  • The relationship of the presenting part to an
    imaginary line drawn b/t the ischial spines of
    the pelvis (see picture on slide 31)

Fetal Position
  • Parts of notation
  • Bony prominence
  • Anterior or posterior
  • Left or right side of maternal abdomen
  • Bony parts presenting
  • Occiput (cephalic presentation, vertex)
  • Mentum (chin, face presentation, hyperextended
    fetal head)
  • Sacrum (breech)
  • Acromium process (shoulder presentation)

Passenger Position
  • Position refers to the relationship of a landmark
    on the presenting fetal part to the anterior,
    posterior or sides left or right of pelvis.
  • Draw four imaginary quadrants left anterior,
    right anterior, left posterior and right
    posterior this designates where the presenting
    part is directed. Next note the
  • Landmarks
  • Occiput
  • Mentum
  • Sacrum
  • Acromion process

Passenger Position
Other presentations
LOA Left Occiput Anterior
Ways to assess fetal Position
  • Leopold Maneuvers
  • Vaginal exam
  • Auscultation of fetal heart tones

Assessing Fetal Position
  • Leopold Maneuvers helps in assessing fetal heart
  • four assessment parameters
  • Fundal exam
  • Fetal exam (lie)
  • Pelvic palpation for vertex presentation
  • Position of fetal body
  • Vaginal exam
  • Helps to determine presentation and position
  • Auscultation of fetal heart tones
  • Heard best thru fetus back in vertex and breech
    position, heard thru chest in transverse lie

PowersPhenomena of Labor
  • Causes of Labor unknown (several theories)
  • Progesterone theory
  • Progesterone produced by the placenta relaxes
    uterine smooth muscle by interfering with
    conduction of impulses from one cell to the next
  • By the end of pregnancy progesterone availability
    decreases this is associated with a yet unknown
    antiprogesten that inhibits the relaxant effect
    on the uterus
  • Oxytocin theory
  • Oxytocin stimulates uterine contractions by
    acting directly on the myometrium (increasingly
    sensitivity to oxytocin) and indirectly increases
    production of prostoglandins initiating labor

PowersPhenomena of Labor
  • Fetal cortisol
  • Corticotropin releasing hormone (CRH)
  • CRH concentration increases throughout pregnancy
    with sharp increase at term gestation
  • CRH stimulates synthesis of prostoglandin F and E
    by amnion cells
  • Secretion of cortical steroids triggers mechanism
    of labor by slowing production of progesterone by
    the placenta and stimulates the release of
  • Prostaglandin
  • Acts on uterine muscle to initiate labor and can
    also stimulate oxytocin and the response to

PowerUterine Contractions
  • Parts of a contraction
  • Primary force is uterine muscular contraction
    which causes the changes in the 1st stage of
  • Uterine contractions are rhythmic tightenings and
    shortenings of the uterine muscles during labor
  • Each ctx has 3 phases
  • Increment-the building up (longest phase)
  • Acme (peak)
  • Decrement-the letting up
  • The relaxation b/t ctx restores uteroplacental
  • Duration
  • Is measured from beginning of ctx to completion
    of ctx
  • Frequency
  • Refers to the time b/t the beginnings of one ctx
    to the beginning of the next ctx

PowerUterine Contractions
  • Intensity
  • Refers to the strength of the ctx during the acme
    of the ctx
  • Examined by palpation
  • Mild, moderate, or strong
  • Measured directly with IUPC
  • External monitoring measures frequency and
  • Variables affecting external monitoring include
    maternal weight, and position of monitor
  • Internal monitoring measures frequency, duration,
    and intensity

Stages of Labor
  • Duration of labor
  • Primipara (up to 20 hours total)
  • 1st stage
  • Latent phase lasts 8.6 hours
  • Active phase lasts 4.6 hours
  • Transition lasts 3.6 hours
  • 2nd stage lasts up to 3 hours
  • 3rd stage lasts 10 min
  • Multipara (6 hours less than first pregnancy
    labor experience)
  • 1st stage
  • Latent 5.3 hours
  • Active 2.4 hours
  • Transition variable
  • 2nd stage 30 min to 0 min
  • 3rd stage 3 min.

Stages of Labor
  • Terms related to labor
  • Effacement
  • The drawing up of the internal os of the cervical
    canal into the uterine side walls
  • The cervix changes progressively from a long and
    thick structure to a structure that is tissue or
    paper thin
  • Primip effacement usually preceeds dilation

Terms related to labor
  • Dilatation
  • Cervical os and cervical canal widen from lt 1cm
    to approx 10 cm allowing for delivery
  • The pt. is complete or 10 cm when the cervix is
    no longer palpable
  • Station
  • The ischial spines mark the narrowest diameter
    through which the fetus must pass.
  • The ischial spines are used as a landmark to
    establish a station of zero.
  • If the presenting part is higher than the ischial
    spines a negative number is assigned, beginning
    with -5 at the inlet of the pelvis and going
    through 4 at the outlet, moving from a negative
    station to zero to a positive station

Cervical Effacement Dilatation
Passenger Station
  • How is mom coping with labor?
  • What is her pain threshold?
  • What kind of support does she have?
  • What is her level of knowledge about the labor
    and delivery process?

Premonitory signs of Labor
  • What sx might mom exhibit that say labor is
  • Lightening (descent of fetal head into the
  • Primip 10-14 days before delivery
  • Multip during labor
  • Cervical ripening (the softening of the cervix)
  • Bloody show (pressure of baby on cervix causes
    microcapillaries to rupture)
  • Blood mixes with mucus
  • Braxton Hicks Contractions (ctx that do not cause
    cervical change)
  • SROM (gush of fluid collects in the vagina)
  • ? energy/nesting syndrome (24-48 hours before
  • Weight loss of 1-3 kg (2.2-6.6lbs)
  • ? frequency of urination
  • diarrhea

True vs. False Labor (p. 541, Olds)
True False
Stages of Labor
  • Stage 1 (closed?complete)
  • Latent phase (0 ? 3 cms)
  • Ctx every 5-30 min, mom feels anxious excited
  • Active phase (4 ?7 cms)
  • Ctx every 2-5 min, mom focused on self
  • Transition phase (8 ? 10 cms)
  • Ctx every 2-3 min, mom is irritable, feels urge
    to push
  • Stage 2 (complete?birth)
  • Expulsion stage, baby crowns, stronger urge to
    push, feels good to push
  • Stage 3 (birth?placenta)
  • Placental stage
  • Rise of uterus in abdomen descent of cord outside
    vagina, sudden gush of blood, takes 5-30 min.
  • Stage 4 (recovery phase 1 hr p birth)
  • Blood loss 250-500cc
  • Potential time for post partum hemorrhage, assess
    vitals, lochia and fundal height every 15 min,
    then 30 min x2, then 60 min x2.

Mechanisms of Labor also known as Cardinal
Movements (pp. 543-545)
  • Engagement descent
  • 4 forces
  • Pressure of amniotic fluid
  • Direct pressure of the fundus of the uterus on
    the breech of the fetus
  • Contraction of the abdominal muscles
  • Extension and straightening of the fetal body
  • Flexion
  • Fetal head meets resistance from pelvic floor
    causing chin to flex downward to chest
  • Internal rotation to OA position
  • Head rotates to fit pelvic cavity by rotating the
    occiput from left to right and sagittal sutures
    align with the anteriorposterior pelvic diameter.

Mechanisms of Labor also known as Cardinal
  • Extension
  • Crowing occurs as the fetal head meets resistance
    from pelvic floor allowing fetal head to pass
    under symphysis pubis
  • Restitution
  • neck is twisted (b/c of internal rotation) once
    free turns head to one side
  • External rotation
  • Shoulders in anteroposterior position, the head
    is turned farther to one side to assist in birth
  • Expulsion
  • After external rotation the anterior shoulder
    meets the undersurface of pubis and slips under,
    shoulders are born then body.

Systemic Response to Labor
  • Cardiac output ? 10-15 in 1st stage, 30-50 in
    2nd stage
  • Position change affects co, side lying increases
    co by 22
  • BP systolic ? during contraction in 1st stage
    systolic diastolic ? during contractions in 2nd
  • WBC ? 25-30000 during labor due to increase in
    neutrophils resulting from a physiologic response
    to stress
  • Respiratory rate ? due to increase in oxygen
    demand and consumption
  • Metabolic acidosis uncompensated by resp.
    alkalosis occurs b/c of pushing (balance is
    reversed in 4th stage)

Systemic response (contd)
  • Temperature may ? slightly, especially if pt. is
  • Proteinuria of 1 may occur
  • Occurs in 1/3 of women in labor
  • Gastric motility is ?
  • Emptying is delayed (risk for aspiration)

Causes of Pain in Labor
  • First stage
  • Pain arises from dilation and stretching of
  • Hypoxia of uterine muscle cell during ctx
  • Stretching of the lower uterine segment
  • Pressure on adjacent structures
  • Second Stage
  • Hypoxia on contracting uterine muscle cells
  • Distention of the vagina and perineum
  • Pressure on back, buttocks, thighs
  • Pudendal nerve transmits nerve impulses for pain

Causes of Pain in Labor
  • 3rd stage
  • Uterine contractions
  • Cervical dilation as placenta is expelled
  • Factors affecting pain
  • Culture
  • Fatigue
  • Previous experiences with pain
  • anxiety

Every new birth is truly a miracle!