Physiology of labor

Obstetrics 1
Lecture
2023-2024
 Learning Objectives

At the end of the lecture, the student should be able to:

1. Describe the phases of parturition

2. Describe the physiologic and biochemical process that

regulate parturition
What is labor?

-uterine contractions that bring about cervical dilatation and

effacement of the cervix.

-it begins with the onset of uterine contractions and ends with the
delivery of the newborn and expulsion of placenta
What initiates labor?

Loss of Synthesis of Mature fetus

maintenance inducing
factors factors
What initiates labor?

Loss of Synthesis of Mature fetus

maintenance inducing
factors factors
1. Maternal and fetal 1. Prostaglandins
compartments
2. Cervix
3. Steroids and
prostaglandins from
placenta
What initiates labor?

Loss of Synthesis of Mature fetus

maintenance inducing
factors factors
1. Maternal and fetal 1. Prostaglandins
compartments
2. Cervix
3. Steroids and
prostaglandins from
placenta
Maternal and fetal compartments
Maternal and fetal compartments
Myometrium

- Composed of bundles of smooth muscles surrounded by

connective tissue
Advantages:
- Readily adaptable to environmental changes
- Greater degree of smooth muscle shortening with
contractions
- Forces can be exerted in multiple directions
- Plexiform arrangement
Maternal and fetal compartments
Decidua

-composed of stromal cells and

maternal immune cells
-serves to maintain the
pregnancy by
immunoregulatory functions

At the end of pregnancy:

-decidual activation ensues

Maternal and fetal compartments
Chorion

-protective tissue layer

-provides immunological acceptance
-Enriched with enzymes that inactivate uterotonins:
-Prostaglandin dehydrogenase
-oxytocinase
-enkephalinase
Maternal and fetal compartments
Amnion

-provides tensile strength to resist membrane tearing and rupture

-avascular
-highly resistant to penetration by leukocytes, microorganisms
and neoplastic cells
-synthesizes prostaglandins
What initiates labor?

Loss of Synthesis of Mature fetus

maintenance inducing
factors factors
1. Maternal and fetal 1. Prostaglandins
compartments
2. Cervix
3. Steroids and
prostaglandins
CERVIX

1. Maintains barrier function to

protect the reproductive
tract from infection
2. Maintains cervical
competence despite
greater gravitational forces
as uterus grows
3. Orchestration of
extracellular matrix
changes that allow
progressively greater tissue
compliance
Clinical Correlation

Cervical incompetence
What initiates labor?

Loss of Synthesis of Mature fetus

maintenance inducing
factors factors
1. Maternal and fetal 1. Prostaglandins
compartments
2. Cervix
3. Steroids and
prostaglandins
 Role of sex steroids

ESTROGEN AND PROGESTERONE

-are components in the maintenance of uterine quiescence
High P:E ratio maintains pregnancy
Low P:E ratio initiates parturition
Clinical correlation:

Drugs in Obstetrics:

Progesterone receptor antagonists: Mifepristone, onapristone will

promote some features of parturition

Progesterone
 Role of PROSTAGLANDIN

Myometrial response stem from

a balance between synthesis
and metabolism

In late pregnancy:
-amnionic prostaglandin
biosytheis is increased
Phospoholipase A2 and
PGHS-2 show greater activity
Phases of Parturition
Phases of Parturition

Clinical Stages
of labor
 Phase 1 of Parturition: UTERINE
QUIESCENCE AND CERVICAL SOFTENING

•
 Phase 1 of Parturition: UTERINE
QUIESCENCE AND CERVICAL SOFTENING
Uterine Quiescence and Cervical Softening
• Uterine Quiescence
• 95% of the pregnancy
• smooth muscle tranquility with maintenance of cervical structural
integrity
• uterus initiates extensive changes in its size and vascularity to
accommodate the pregnancy and prepare for uterine contractions
• Braxton Hicks contractions or false labor
Quiescence is achieved by:

1.Diminished intracellular crosstalk and

reduced intracellular Ca levels
2. Ion channel regulation of cell
membrane potential
3. Activation of the uterine endoplasmic
reticulum stress-unfolded protein
response
4. Uterotonin degradation
Quiescence is achieved by:

1.Diminished intracellular crosstalk and

reduced intracellular Ca levels
2. Ion channel regulation of cell
membrane potential
3. Activation of the uterine endoplasmic
reticulum stress-unfolded protein
response
4. Uterotonin degradation
Quiescence is achieved by:

1.Diminished intracellular crosstalk and

reduced intracellular Ca levels
2. Ion channel regulation of cell
membrane potential
3. Activation of the uterine endoplasmic
reticulum stress-unfolded protein
response
4. Uterotonin degradation
Endoplasmic reticulum stress response

Caspase 3
Quiescence is achieved by:

1.Diminished intracellular crosstalk and

reduced intracellular Ca levels
2. Ion channel regulation of cell
membrane potential
3. Activation of the uterine endoplasmic
reticulum stress-unfolded protein
response
4. Uterotonin degradation
PGDH- Prostaglandins
Enkephalinase-Endothelins
Oxytocinase-Oxytocin
Diamine oxidase-histamine
CAtechol-o-methytransferase-Cathecholamin
es
 Phase 1 of Parturition

CERVICAL SOFTENING
-greater tissue compliance, but remains firm and unyielding
-results from greater vascularity, cellular hypertrophy and
hyperplasia and slow, progressive compositional or structural changes of
the extracellular matrix

COLLAGEN
-main structural protein in the cervix
 Phase 2 of Parturition: PREPARATION
FOR LABOR
•
 Phase 2 of Parturition: PREPARATION
FOR LABOR
Preparation for Labor

• uterine awakening or activation

• progression of uterine changes during the last 6 to 8 weeks of

pregnancy
 Phase 2 of Parturition

Preparation for Labor

• Myometrial changes
• contraction-associated proteins (CAPs)
• oxytocin receptor
• prostaglandin F receptor control contractility

• connexin 43
• increased uterine irritability and responsiveness to uterotonins –
agents that stimulate contractions
 Phase 2 of Parturition

Preparation for Labor

• Myometrial changes
• formation of the lower uterine
segments from the isthmus
• the head descends to or
even through the pelvic inlet
- lightening
• Oxytocin receptors rises, and
is regulated by progesterone
and estradiol
 Phase 2 of Parturition

Preparation for Labor

• Cervical Ripening
• transition from softening to ripening phase begins weeks or days
before onset of contractions
• total amount of proteoglycans and glycosaminoglycans within the
matrix are altered
 Phase 2 of Parturition

Preparation for Labor

• Endocervical Epithelia
• in pregnancy, cells proliferate such that endocervical glands occupy
a significant percentage of cervical mass
• in mice, these cells may also aid cervical remodeling by regulating
tissue hydration and maintenance of barrier function
 Phase 2 of Parturition

Preparation for Labor

• Cervical Connective Tissue

• Collagen – largely responsible for the structural disposition of the
cervix
• collagen fibrils interact with small proteoglycans such as decorin or
biglycan, as well as matricellular proteins such as thrombospondin 2
• these interactions determine fibril size, packing, and organization
 Phase 2 of Parturition

• Glycosaminoglycans (GAGs)

• high-molecular-weight polysaccharides that complex with proteins to

form proteoglycans

• Hyaluronan (HA) – increased during ripening

• increase viscoelasticity
• proinflammatory properties, increased during labor and in
the pueperium
 Phase 2 of Parturition

• Inflammatory changes
• stromal invasion with inflammatory cells
• “ inflammatory process”
Fetal contributions

Uterine stretch

Fetal endocrine cascade

Surfactant protein A

Fetal membrane senescence

Clinical Correlations:

Induction of cervical ripening

-progesterone antagonists

Preterm labor

Adrenal hypoplasia

Brain anomalies

Renal agenesis, pulmonary hypoplasia

 Phase 3 of Parturition: LABOR

•
 Phase 3 of Parturition: LABOR

•
• Three Clinical Stages
• Stage of cervical effacement
and dilatation
• Stage of fetal expulsion
• Stage of placental separation
and expulsion
 Phase 3 of Parturition

• First Stage of Labor

• painful uterine smooth muscular contractions - involuntary
1. hypoxia of the contracted myometrium

2. compression of nerve ganglia in the cervix and lower uterus by contracted

interlocking muscle bundles*
3. cervical stretching during dilatation

4. stretching of the peritoneum overlying the fundus

 - Intervals between contractions narrows gradually from 10
minutes at the onset of first stage labor to as little as 1 minute
or less in second stage

-periods of relaxation is essential for fetal welfare

-contraction ranges from 30-90 seconds and averages 1 minut

Ferguson reflex

– mechanical stretching of the

cervix enhances uterine activity
- release of oxytocin
stripping – increase in blood levels of
prostaglandin F2α
metabolite (PGFM)
 Phase 3 of Parturition

• First Stage of Labor

• Distinct Lower and Upper Uterine Segments
 Phase 3 of Parturition

Changes in Uterine Shape During Labor

• elongation of the ovoid uterine shape with
concomitant decrease in horizontal diameter.
• increase fetal axis pressure – straighten the
fetal vertebral column
• longitudinal muscle fibers are drawn taut –
lower segment and cervix of the uterus are left
flexible
 Phase 3 of Parturition

• Ancillary Forces in Labor

• the most important force in fetal
expulsion – maternal intraabdominal
pressure (pushing)
 Phase 3 of Parturition

• Cervical Changes
• Cervical dilatation
• Cervical effacement
Cervical Dilatation

• because the lower segment

and cervix have lesser
resistance during a contraction,
a centrifugal pull is exerted on
the cervix and creates cervical
dilatation
• as uterine contractions cause
pressure on the membranes,
the hyrdostatic action of the
amnionic sac in turns dilates
the cervical canal like a wedge
cervical effacement
– “obliteration” or “taking
up” of the cervix
• shortening of the
cervical canal from
approximately 2 cm to
a mere circular orifice
with almost paper-thin
edges
• causes expulsion of
the mucous plug as the
cervical canal is
shortened
 Phase 3 of Parturition

• Second Stage of Labor

• Fetal Descent
• Station – descent of the fetal biparietal diameter in relation to a line drawn
between maternal ischial spines
 Phase 3 of Parturition

• Second Stage of Labor

• Pelvic Floor Changes During Labor
• most important structures
• levator ani muscles – pubovisceral, puborectalis,
iliococcygeus muscles
• in pregnancy, undergo hypertrophy
• on contraction, draws the rectum and vagina forward
and upward in the direction of the symphysis pubis
 Phase 3 of Parturition

• Third Stage of Labor

• Delivery of Placenta and Membranes
• the uterus spontaneously contracts around its diminishing contents
The sudden diminution in uterine size is inevitably accompanied by a decrease in
the area
of the placental implantation site
Postpartum, membranes are thrown up into folds as the uterine cavity decreases
in size
 Phase 4 of Parturition: PUERPERIUM

•
 Phase 4 of Parturition: PUERPERIUM

• First Hour After Delivery of Placenta

• the myometrium remains in a state of rigid and persistent contraction
and retraction
• this directly compresses large uterine vessels and allows thrombosis
of their lumens to prevent hemorrhage
• typically augmented by uterotonics
 SUMMARY

2 General Theories:

1. Functional loss of pregnancy maintenance factors

2. Synthesis of factors that induce parturition

3. Mature fetus is the source of the initial signal for parturition
commencement.
Phase 1 Phase 2 Phase 3 Phase 4

• Fail-safe System • Functional • increased • A complicated series

to Maintain Progesterone uterotonin of repair processes
Withdrawal are initiated to resolve
Uterine production would inflammatory
Quiescence • mediated through follow once phase responses and
several 1 is suspended remove
• actions of estrogen mechanisms and uterine phase glycosaminoglycans,
proteoglycans, and
and progesterone via • progesterone-recept 2 processes are structurally
intracellular receptors or activity is implemented compromised collagen
decreased late in
• myometrial cell gestation
• uterotonins:
plasma
membrane • The fetus may provide • oxytocin • matrix and cellular
receptor-mediate the initial signal components required
d increases in
cAMP • Uterine stretch • prostaglandins for complete uterine
involution are

• the generation of • fetal adrenals produce • serotonin synthesized, and the

dense connective
cortisol tissue and structural
cGMP • histamine integrity of the cervix
• placental CRH
• other systems, regulates the adrenal
glands and in turn
• PAF are reformed

including
modifications in
cortisol can stimulate
CRH production
• angiotensin II
myometrial cell
ion channels • result in an increase in
estrogens which would
shift the
progesterone-estrogen
ratio and promote the
expression of CAPs
leading to a loss of
myometrial quiescence
•
 For book and journal references

• Willliams Obstetrics, 25th edition,

• Chapter 21 (Physiology of Labor)
Thank you for your attention!