Physiology of Labor: Onset and Key Hormonal Changes
ONSET AND PHYSIOLOGY OF
LABOR
Dr. Hana Alzamil
Objectives:
• Define labor/labour (parturition).
• Recognize the factors triggering the onset of labor.
• Describe the hormonal changes that occur before
and during labor.
• Describe the phases of uterine activity during
pregnancy and labor.
• Know the clinical stages of labor.
Parturition
Definition
Uterine contractions that lead to
expulsion of the fetus to extrauterine
environment
Towards the end of pregnancy the
uterus become progressively more
excitable and develops strong rhythmic
contractions that lead to expulsion of the
fetus.
Small group activity
Does non pregnant uterus contract?
Parturition
Uterus is spontaneously active.
Spontaneous depolarization of pacemaker
cells (Telocytes).
Gap junctions spread depolarization
Exact trigger is unknown
Hormonal changes
Mechanical changes
Telocytes
Hormonal changes
Estrogen & Progesterone
Progesterone inhibit uterine contractility
Estrogen stimulate uterine contractility
From 7
th
month till term
Progesterone secretion remain constant
Estrogen secretion continuously
increase
Increase estrogen/progesterone ratio
Hormonal changes
Progesteron
▼ GAP junctions
▼ Oxytocin
receptor
▼Prostaglandins.
▲ Resting mem.
Potential
P
Estrogen
▲ GAP junctions
▲ Oxytocin
receptors.
▲ Prostaglandins
E
Hormonal changes
Oxytocin
Dramatic ▲of oxytocin receptors (200
folds)
gradual transition from passive relaxed to
active excitatory muscle
(↑responsiveness).
Increase in Oxytocin secretion at labor
Oxytocin increase uterine contractions by
Directly on its receptors
Indirectly by stimulating prostaglandin
production
Hormonal changes
Prostaglandins
Central role in initiation & progression of human
labour
Locally produced (intrauterine)
Oxytocin and cytokines stimulate its production
Prostaglandin stimulate uterine contractions by:
Direct effect:
Through their own receptors
Upregulation of myometrial gap junctions
Indirect effect:
Upregulation of oxytocin receptors
Parturition
Mechanical changes
Stretch of the uterine muscle
Increases contractility
Fetal movements
Multiple pregnancy
Stretch of the cervix
Increases contractility (reflex)
Membrane sweeping & rupture
Fetal head
Positive feedback mechanism
Initiation of Labor
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Phases of parturition
Phase 0
Pregnancy: uterus is relaxed (quiescent)
Phase 1
Activation
Phase 2
Stimulation: stage 1& stage 2
Phase 3 = stage 3
Delivery of the placenta and uterine
involution
Uterine Activity During Pregnancy
21
Inhibitors
•
Progesterone
•
Prostacycline
•
Relaxin
•
Nitric Oxide
•
Parathyroid
hormone-related
peptide
Quiescence
Phase 0
Uterotrophins
•
Estrogen
•
Progesterone
•
Prostaglandins
•
CRH
Activation
Phase 1
Uterotonins
Involution
•
Prostaglandins
•
Oxytocin
•
Oxytocin
Stimulation
Involution
Phase 2
Phase 3
Phases of parturition
Phase 0 (pregnancy)
Increase in cAMP level
Increase in production of
Prostacyclin (PGI
2
) cause uterine
relaxation
Nitric oxide (NO)
cause uterine relaxation
Adapted from Smith, 2007
Phases of parturition
P
h
a
s
e
1
(
a
c
t
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v
a
t
i
o
n
)
Occurs in third trimester
Promote a switch from quiescent to active
uterus
Increase excitability & responsiveness by
Increase expression of gap junctions
Increase G protein-coupled
receptors
Oxytocin receptors
PGF2 alpha receptors
Phases of parturition
P
h
a
s
e
2
(
s
t
i
m
u
l
a
t
i
o
n
)
Occurs in last 2-3 gestational weeks
Increase in synthesis of uterotonins
Cytokines
Prostaglandins
Oxytocin
Includes 2 stages:
Stage 1
Stage 2
Phases of parturition
P
h
a
s
e
3
(
u
t
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r
i
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)
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Occurs in 4-5 weeks after delivery
Lactation helps in complete involution
Mechanism of parturition
Contractions start at the fundus and
spreads to the lower segment
The intensity of contractions is strong at
the fundus but weak at the lower segment
In early stages: 1 contraction/ 30 minuets
As labor progress: 1 contraction/ 1-3
minutes
Abdominal wall muscles contract
Rhythmical contractions allows blood flow
Onset of labor
During pregnancy
Periodic episodes of weak and slow
rhythmical uterine contractions (Braxton
Hicks) 2
nd
trimester
Towards end of pregnancy
Uterine contractions become progressively
stronger
Suddenly uterine contractions become very
strong leading to:
Cervical effacement and dilatation
Stages of Labor
Dilation (stage 1)
Cervix becomes dilated
Full dilation is 10 cm
Uterine contractions begin and increase
Cervix softens and effaces (thins)
The amnion ruptures (“breaking the water”)
Longest stage at 6-12 hours
Cervical effacement and dilatation
Stages of Labor
F
(
1
o
f
3
)
Stages of Labor
Expulsion (stage 2)
Infant passes through the cervix and
vagina
Can last as long as 2 hours, but typically
is 50 minutes in the first birth and 20
minutes in subsequent births
Normal delivery is head first (vertex
position)
Breech presentation is buttocks-first
Stages of Labor
F
i
g
u
r
e
1
6
.
2
0
(
2
o
f
3
)
Stages of Labor
Placental stage (stage 3)
Delivery of the placenta
Usually accomplished within 15 minutes
after birth of infant
Afterbirth—placenta and attached fetal
membranes
All placental fragments should be
removed to avoid postpartum bleeding
Stages of Labor
F
i
g
u
r
e
1
6
.
2
0
(
3
o
f
3
)
New arrival
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Labor, or parturition, involves uterine contractions leading to the fetus's expulsion. Factors triggering labor include hormonal changes like increased estrogen and decreased progesterone, which stimulate uterine muscle activity. Telocytes play a role in spontaneous uterine activity, while oxytocin and prostaglandins contribute to contractions during labor. Understanding the phases of uterine activity and hormonal shifts is crucial in comprehending the onset and progression of labor.
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Presentation Transcript
ONSET AND PHYSIOLOGY OF LABOR Dr. Hana Alzamil
Objectives: Define labor/labour (parturition). Recognize the factors triggering the onset of labor. Describe the hormonal changes that occur before and during labor. Describe the phases of uterine activity during pregnancy and labor. Know the clinical stages of labor.
Parturition Definition Uterine contractions that lead to expulsion of the fetus to extrauterine environment Towards the end of pregnancy the uterus become progressively more excitable and develops strong rhythmic contractions that lead to expulsion of the fetus.
Small group activity Does non pregnant uterus contract?
Parturition Uterus is spontaneously active. Spontaneous depolarization of pacemaker cells (Telocytes). Gap junctions spread depolarization Exact trigger is unknown Hormonal changes Mechanical changes
Hormonal changes Estrogen & Progesterone Progesterone inhibit uterine contractility Estrogen stimulate uterine contractility From 7th month till term Progesterone secretion remain constant Estrogen secretion continuously increase Increase estrogen/progesterone ratio
Hormonal changes Estrogen GAP junctions Oxytocin receptors. Prostaglandins E Progesteron GAP junctions Oxytocin receptor Prostaglandins. Resting mem. Potential P
Hormonal changes Oxytocin Dramatic of oxytocin receptors (200 folds) gradual transition from passive relaxed to active excitatory muscle ( responsiveness). Increase in Oxytocin secretion at labor Oxytocin increase uterine contractions by Directly on its receptors Indirectly by stimulating prostaglandin production
Hormonal changes Prostaglandins Central role in initiation & progression of human labour Locally produced (intrauterine) Oxytocin and cytokines stimulate its production Prostaglandin stimulate uterine contractions by: Direct effect: Through their own receptors Upregulation of myometrial gap junctions Indirect effect: Upregulation of oxytocin receptors
Mechanical changes Stretch of the uterine muscle Increases contractility Fetal movements Multiple pregnancy Stretch of the cervix Increases contractility (reflex) Membrane sweeping & rupture Fetal head Positive feedback mechanism
Initiation of Labor Baby moves deeper into mother s birth canal Figure 16.19, step 1
Initiation of Labor Baby moves deeper into mother s birth canal Pressoreceptors in cervix of uterus excited Figure 16.19, step 2
Initiation of Labor Baby moves deeper into mother s birth canal Afferent impulses to hypothalamus Pressoreceptors in cervix of uterus excited Figure 16.19, step 3
Initiation of Labor Hypothalamus sends efferent impulses to posterior pituitary, where oxytocin is stored Baby moves deeper into mother s birth canal Afferent impulses to hypothalamus Pressoreceptors in cervix of uterus excited Figure 16.19, step 4
Initiation of Labor Hypothalamus sends efferent impulses to posterior pituitary, where oxytocin is stored Posterior pituitary releases oxytocin to blood; oxytocin targets mother s uterine muscle Baby moves deeper into mother s birth canal Afferent impulses to hypothalamus Pressoreceptors in cervix of uterus excited Figure 16.19, step 5
Initiation of Labor Hypothalamus sends efferent impulses to posterior pituitary, where oxytocin is stored Posterior pituitary releases oxytocin to blood; oxytocin targets mother s uterine muscle Uterus responds by contracting more vigorously Baby moves deeper into mother s birth canal Afferent impulses to hypothalamus Pressoreceptors in cervix of uterus excited Positive feedback mechanism continues to cycle until interrupted by birth of baby Figure 16.19, step 6
Phases of parturition Phase 0 Pregnancy: uterus is relaxed (quiescent) Phase 1 Activation Phase 2 Stimulation: stage 1& stage 2 Phase 3 = stage 3 Delivery of the placenta and uterine involution
Uterine Activity During Pregnancy 21 Inhibitors Uterotrophins Estrogen Progesterone Prostaglandins CRH Uterotonins Prostaglandins Oxytocin Involution Oxytocin Progesterone Prostacycline Relaxin Nitric Oxide Parathyroid hormone-related peptide Quiescence Phase 0 Activation Phase 1 Stimulation Phase 2 Involution Phase 3
Phases of parturition Phase 0 (pregnancy) Increase in cAMP level Increase in production of Prostacyclin (PGI2) cause uterine relaxation Nitric oxide (NO) cause uterine relaxation Adapted from Smith, 2007
Phases of parturition Phase 1 (activation) Occurs in third trimester Promote a switch from quiescent to active uterus Increase excitability & responsiveness by Increase expression of gap junctions Increase G protein-coupled receptors Oxytocin receptors PGF2 alpha receptors
Phases of parturition Phase 2 (stimulation) Occurs in last 2-3 gestational weeks Increase in synthesis of uterotonins Cytokines Prostaglandins Oxytocin Includes 2 stages: Stage 1 Stage 2
Phases of parturition Phase 3 (uterine involution) Pulsatile release of oxytocin Delivery of the placenta Involution of the uterus Occurs in 4-5 weeks after delivery Lactation helps in complete involution
Mechanism of parturition Contractions start at the fundus and spreads to the lower segment The intensity of contractions is strong at the fundus but weak at the lower segment In early stages: 1 contraction/ 30 minuets As labor progress: 1 contraction/ 1-3 minutes Abdominal wall muscles contract Rhythmical contractions allows blood flow
Onset of labor During pregnancy Periodic episodes of weak and slow rhythmical uterine contractions (Braxton Hicks) 2nd trimester Towards end of pregnancy Uterine contractions become progressively stronger Suddenly uterine contractions become very strong leading to: Cervical effacement and dilatation
Stages of Labor Dilation (stage 1) Cervix becomes dilated Full dilation is 10 cm Uterine contractions begin and increase Cervix softens and effaces (thins) The amnion ruptures ( breaking the water ) Longest stage at 6-12 hours
Stages of Labor F (1 of 3)
Stages of Labor Expulsion (stage 2) Infant passes through the cervix and vagina Can last as long as 2 hours, but typically is 50 minutes in the first birth and 20 minutes in subsequent births Normal delivery is head first (vertex position) Breech presentation is buttocks-first
Stages of Labor Figure 16.20 (2 of 3)
Stages of Labor Placental stage (stage 3) Delivery of the placenta Usually accomplished within 15 minutes after birth of infant Afterbirth placenta and attached fetal membranes All placental fragments should be removed to avoid postpartum bleeding
Stages of Labor Figure 16.20 (3 of 3)
