seminar on physiology of labor

1. PHYSIOLOGY OF LABOR
BY DR. WONDMENEH K
May 2021
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2. Outline
Definition
Phases of myometrial activity
Physiology Labor Initiation and important hormones
Mechanism of labor
Stages of labor
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3. Definition
• Labor is defined as the process by which the fetus is
expelled from the uterus.
• requires regular, effective contractions that lead to
dilation and effacement of the cervix.
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4. Physiology of Labor Initiation
• The physiology of labor initiation has not been
completely elucidated
• putative mechanisms have been well reviewed
• Labor initiation is species-specific
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5. Theories of labor initiation
• Fetal cortisol theory- mature fetus secretes cortisol
which initiates labor
• Oxytocin theory- secretion of oxytocin from
posterior hypophysis initiates labor
• Prostaglandin theory- release of prostaglandins
from fetal membranes initiates labor
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6. PHASES OF MYOMETRIAL ACTIVITY
• Four physiologic phases
Phase 0:quiescence
Phase 1:activation phase
Phase 2: stimulatory phase
Phase 3: involution
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7. Phase 0: Quiescence
• represents the time in utero before labor begins
when uterine activity is suppressed by the action
• Progesterone
• Prostacyclin
• Relaxin
• Parathyroid hormone-related peptide
• Nitric oxide
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8. Phase 1: Activation phase
• As term approaches the uterus becomes activated
in response to uterotropins, such as estrogen.
• estrogen begins to facilitate expression of
myometrial receptors for prostaglandins (PGs) and
oxytocin, which results in ion channel activation and
increased gap junctions.
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9. Phase 2: Stimulatory phase
• The "primed" uterus can be stimulated to contract by
the action of uterotonic agonists:
• prostaglandins E2 and F2 alpha
• oxytocin
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10. Phase 3: involution
• occurs after delivery
• is mediated primarily by oxytocin.
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12. HORMONES INVOLVED IN THE
PARTURITION CASCADE
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13. Prostaglandins
• are important in initiation of labor probably
common to all species
• Are considered as uterotonins, but some acts as
smooth muscle relaxants
• Includes:
• PGI2
• PGE2
• PGF2α
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14. Progesterone
• Is necessary for early pregnancy maintenance.
• In many species, progesterone withdrawal, directly
precedes progression of phase 1 into phase 2 of
parturition
• The role of progesterone in late pregnancy is not well
defined in humans.
• Progesterone withdrawal is not a prerequisite for labor
in humans.
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15. Estrogen
• Placenta is the primary source of biosynthesis
during pregnancy.
• Estrogens act by upregulating of:
• Myometrial gap junctions and
• Uterotonic receptors (including L-type calcium
channels and oxytocin receptors).
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16. Oxytocin
• Most potent endogenous uterotonic agent
• circulating levels of oxytocin do not change
significantly during pregnancy or prior to the onset
of labor.
• myometrial oxytocin receptor concentrations
increase during pregnancy, reaching a maximum
during early labor
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17. Oxytocin ctd..
• Oxytocin acts by 2 mechanism
• Directly on oxytocin receptors
• Indirectly through stimulation of prostaglandin
production
• It is unlikely that oxytocin provides the trigger for
the initiation of labor, but its release during labor
facilitates delivery of the fetus and placenta.
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18. Relaxin
• is a member of the insulin-like growth factor family
of proteins.
• highest at 8 to 12 weeks of gestation and thereafter
decline to low levels, which persist until term.
• The primary source : the corpus luteum.
• appears to act indirectly to promote myometrial
relaxation by stimulating myometrial prostacyclin
production.
• Degrades matrix molecules like collagen
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19. Corticotrophin-Releasing Hormones
• Synthesized in the placenta and hypothalamus.
• plasma levels increase dramatically during the final 6 to 8
weeks of pregnancy.
• CRH has no direct inotropic action on myometrium, but
have multiple actions :
• stimulate DHEA-S secretion
• Enhances PG production in the amnion, chorion and
decidua
• Primes myometrium and potentiates the effects of
oxytocin
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20. Corticotrophin-Releasing Hormones
ctd..
• Placental CRH has been proposed to play several
roles in parturition regulation.
• Placental CRH may enhance fetal cortisol
production to provide positive feedback so that the
placenta produces more CRH.
• Rising level of CRH at the end of gestation reflects a
fetal-placental clock
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22. Calcium ion and contractions
• Regulates interaction between actin-myosin
• Has a key role in Ux contraction.
• Many factors regulate CA++ flow into myometrial cells
and from intracellular stores
• Eg hormones like oxytocin,PG,Endothelins,EGF
• Any condition that block Ca ++ influx inhibit Ux
contraction. Eg. CCB for labor inhibition.
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23. Other
• Parathyroid hormone-related peptide
• Luteinizing hormone/human chorionic
gonadotropin
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24. LABOR AS EXAMPLE OF A POSITIVE
FEEDBACK
• During pregnancy, the uterus undergoes periodic
episodes of weak and slow rhythmic contractions
called Braxton Hicks contractions.
• become progressively stronger toward the end of
pregnancy; then they change suddenly, within
hours, to become strong contractions.
• The exact mechanism of this change to a strong
contraction is not known, but the a theary,positive
feedback theory.
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25. • Toward the end of pregnancy, the uterus becomes
progressively more excitable.
• The exact cause of the increased activity of the
uterus is not known, but at least two
• Hormonal changes
• Progressive mechanical changes.
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27. Physiology of labor: contractions
• Uterus contracts through out pregnancy:
• braxton hicks contraction
• Contractions in late third trimester (false labor):
• more frequent but on & off
• Contraction in true labor:
• Regular, frequent, intensity increasing
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28. MECHANISM OF LABOR
• The ability of the fetus to successfully negotiate the
pelvis during labor and delivery depends on the
complex interactions of three variables:
• The Fetus (Passenger)
• The Maternal Pelvis (Passage)
• Uterine Activity (Powers)
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29. CARDINAL MOVEMENTS IN LABOR
• Refer to the changes in position of fetal head during
its passage through the birth canal.
• seven discrete cardinal movements of the fetus are
described: engagement, descent, flexion, internal
rotation, extension, external rotation or restitution,
and expulsion
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30. • The first three steps occur simultaneously.
– Engagement: movement of the presenting part
below the plane of the pelvic inlet
– Descent: movement of the presenting part down
through the curve of the birth canal
– Flexion: placement of the fetal chin on the thorax
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31. • The next four steps occur in order.
– Internal rotation: rotation of the position of the fetal
head in the mid pelvis from transverse to anterior-
posterior
– Extension: movement of the fetal chin away from the
thorax
– External rotation: rotation of the fetal head outside
the mother as the head passes through the pelvic
outlet
– Expulsion: delivery of the fetal shoulders and body
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36. STAGES OF LABOR
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37. First stage of labor
• From onset of regular uterine contractions to
complete cervical dilation.
• is divided into a latent and an active phase.
– Latent phase begins with onset of regular
contractions and ends with the acceleration
– Active phase begins with cervical dilation
acceleration ending with complete cervical dilation.
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38. Second stage of labor
• begins with complete cervical dilation and
ends with delivery of the fetus
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39. Third Stage of labor
• begins with delivery of the fetus and ends with
expulsion of the placenta.
• Signs of third Stage of labor include
– gush of blood vaginally,
– change of the uterus from long to globular
– “lengthening” of the umbilical cord
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42. Updates on the labor curve
• Newer studies (Zhang et al, 2010) based on >60,000
laboring women at 19 U.S. medical centers produce
contemporary labor curves and norms which differ
significantly from the older Friedman data.
• The new data suggest the following:
– Transition from latent to active phase is at 6 cm,
rather than 4 cm.
– Rate of active phase cervical dilation curve is
much slower than previously thought.
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44. References
• Guyton and Hall Textbook of Medical
Physiology
• Kaplan gynecology and obstetrics 2020
• Williams obstetrics 24th e
• Gabbe-steven obstetrics 6th e
• Uptodate 21.2
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45. THANKS
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