The document discusses the physiology of labor in three parts: initiation and hormones, uterine contractions, and stages of labor. Labor initiation involves hormones like progesterone, estrogen, oxytocin, prostaglandins, and corticotropin-releasing hormone from the fetus and placenta. Uterine contractions occur in four phases from quiescence to involution. Labor progresses through three stages from cervical dilation to delivery of the placenta. Cardinal movements describe the fetus's passage through the birth canal during the second stage.

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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