The physiology of labor involves three main phases: 1) Uterine quiescence and cervical softening in which the cervix prepares for labor through changes in vascularity and collagen. 2) Preparation for labor in which cervical ripening and increases in hormones like oxytocin and prostaglandins make the uterus more contractile. 3) Active labor consisting of 3 stages - dilation of the cervix, delivery of the fetus, and delivery of the placenta through uterine contractions and retraction. A combination of hormonal, mechanical, and fetal factors all contribute to initiating the complex process of parturition.

1. PHYSIOLOGY OF LABOR

2. TERMINOLOGY
• LABOUR- The process by which a fetus of viable age is expelled
from the uterus through vagina. It may be spontaneous or
induced.
• DELIVERY – It includes both expulsion of fetus through vagina
at the end of labour and extraction of fetus from the uterus
through cesarean section.
• PARTURITION – It is the process of delivering the fetus and
placenta.
*PARTURIENT is a patient in labor

3. NORMAL LABOUR- IF IT MEETS FOLLOWING
CRITERIA
• PREGNANCY – Singleton
• GESTATION – Term
• PRESENTATION – Vertex
• ONSET – Spontaneous
• DURATION – Without undue delay
• MODE OF DELIVERY – Vaginal
• ASSISTANCE – Minimal (episiotomy)
• FETOMATERNAL COMPLICATIONS – None (healthy mother and baby)

4. WHAT INITIATE LABOR
• The exact cause and mechanism are not known.
• There are three main theories:
• Functional loss of pregnancy maintenance factors
• Synthesis of factors that induce parturition
• The mature fetus is the source of the initial signal for parturition
commencement.
• The principal factors in the initiation of labor are:
• Hormones
• Fetus and placenta
• Mechanical
• It is a combination of these factors which results in the onset of labour.
• It is a gradual process which develops slowly in late pregnancy.
• The placenta and fetal membranes play a major role in the initiation of labour ,

5. 1. HORMONES
A) PROGESTERONE AND OESTROGEN
• Progesterone has an effect on membrane potential opposite to
that of oestrogen.
• Evidence suggests that increased progesterone-to-estrogen
ratio helps in the maintenance of pregnancy and a decline in
the progesterone-to-estrogen ratio initiates for parturition.
• Estrogen also stimulates prostaglandin synthesis and sensitizes
the myometrium gradually, so that it responds to oxytocin or
any other stimuli.

6. 1. HORMONES
B) OXYTOCIN
• The oxytocin sensitivity of myometrium increases in the late
pregnancy due to an increase in the number of oxytocin
receptors.
• Oxytocin levels in the blood increases in labour and are
maximal in the second stage, therefore, important during the
late expulsive stages of labour.
• Oxytocin modifies cell membrane potential, inhibits calcium
binding in the sarcoplasmic reticulum increasing the
concentration of intracellular free calcium which promotes
myometrial contraction.

7. 1. HORMONES
C) PROSTAGLANDINS
• FERGUSON REFLEX – Mechanical stimulation of cervix by the insertion
of a finger or separation of membranes from the lower uterine
segment leads to local secretion of naturally occurring
prostaglandins.
• They can initiate uterine contractions at any stage of pregnancy.
• Prostaglandins are present in the decidua, placenta, liquor amnii and
the fetal membranes. The fetal membranes serve as the site of
storage and release of the prostaglandin precursor, arachidonic acid
which is converted to prostaglandin E and F, which diffuses into the
myometrium and initiates labor.

8. The fetal membranes serve as the site of storage and release of the
prostaglandin precursor, arachidonic acid which is converted to
prostaglandin E and F, which diffuses into the myometrium and

9. 1. HORMONES
D) OTHERS
• LUTEINIZING HORMONE- Receptors are produced more during
pregnancy than in labor.
• Human Chorionoc Gonadotropin (HCG)- It decreases the
myometrial cell gap junctions, frequency and force of
contractions.
• RELAXIN – It helps in cervical softening and inhibition of
myometrial contraction.
• Corticotropin Releasing Hormone (CRH)- It promotes
myometrial quiescence during pregnancy but aids myometrial
contractions with onset of parturition.

10. 2. FETAL CONTRIBUTIONS TO INITIATION
OF PARTURITION
• Fetal adrenals plays a role in the initiation of labor.
• The fetus provides a signal from fetal hypothalamic pituitary-
adrenal axis through fetal adrenal cortisol and
Dehydroepiandrosterone-sulfate (DHEAS) which have effect on
placenta and membrane that eventually transform myometrium
from a quiescent to contractile state.

11. 3. PLACENTAL CONTRIBUTIONS TO
INITIATION OF PARTURITION
• Corticotropin releasing hormone (CRH) identical to maternal and fetal
hypothalamic CRH is synthesized by the placenta. Toward term, CRH
levels rises sharply, peak during labour and fall after delivery..
• Placental CRH plays a role in timing and regulation of parturition.
• CRH augments the contraction-inducing potency of a given dose of
oxytocin in human myometrial strips.
• CRH stimulates fetal adrenal C19-steroid synthesis increasing
substrate for placental aromatization, thus increasing production of
oestrogen

13. Fetal cortisol reduces placental progesterone production.
Production of prostaglandins in decidua and fetal membranes
Increase in the output of oxytocin from the posterior pituitary and
oxytocin increases the synthesis or release of prostaglandins
Oxytocin inhibits the binding of calcium thus increasing its amount in the myom
Decreased progesterone and increase in estrogen and formation of
prostaglandins enhance formation of gap junctions in myometrium

15. 4. MECHANICAL FACTORS
• UTERINE DISTENSION - Uterine stretch due to increasing size of the
fetus induces specific contraction-associated proteins (CAPs),
especially gap junction protein – connexin 43 and of oxytocin
receptors.
• Overstretching of uterus in polyhydramnios and multiple pregnancy
may promote muscle excitability and plays a role in early onset of
labour.
• PRESSURE OF THE PRESENTING PART – Pressure of the presenting part
on the lower segment stretches the lower uterine segment inducing
reflex release of oxytocin and activating prostaglandin activity,
leading to cervical ripening, effacement and start of dilation.

17. PHASES OF
PARTURITI
ON
1. Uterine quiescence and cervical
softening
2. Preparation for labor (cervical ripening
and myometrial changes)
3. Labour
4. Puerperium

19. PHASE 1
UTERINE QUIESCENCE AND CERVICAL
SOFTENING
• Highly irregular and low intensity myometrial contractions
(referred as Braxton Hicks contractions) which do not cause
cervical effacement or dilatation occurs during this phase.
• CERVICAL SOFTENING, first stage of remodeling of cervix
occurs during this phase.
*Structural changes which occurs in cervix during
softening are increased vascularity, stromal as well as glandular
hypertrophy and hyperplasia, compositional changes in
extracellular matrix and increase in collagen solubility.

20. PHASE 2
PREPARATION OF LABOR (PRELABOUR)
• CERVICAL RIPENING – Extensive remodeling in the cervix begins weeks
before the onset of contractions, to facilitate cervical yielding and
dilatation upon contractions.
*Epithelial cells help in tissue hydration through aquaporins
expression.
*Collagen fibrils become less uniform, disorganized, widely spaced,
shortened and degraded.
• MYOMETRIAL CHANGES – It include increase in oxytocin, prostaglandin F
receptors and gap junction proteins (connexin 43) which increases uterine
irritability and responsiveness to uterotonins.
* Formation of lower segment of uterus from lower part of corpus,
isthmus and upper part of cervix occurs. The fetal head often descends to or

21. CERVICAL
RIPENING
COLLAGEN
ARRANGEMENT
• Collagen fibrils
become less
uniform,
disorganized,
widely spaced,
shortened and
degraded in
second phase of
parturition.

22. LIGHTENING

23. PHASE 3 - ACTIVE LABOUR
• Myometrial contraction and cervical dilatation occurs.
• Three stages of labour:
1. First stage – Stage of cervical dilatation. It starts from the onset of true
labor pain and the end is signaled by the full dilatation (10 cm) and
effacement of the cervix.
2. Second stage – Stage of delivery of the fetus. It lasts from full dilatation
of the cervix to the final birth of the baby.
3. Third stage – Stage of delivery of placenta and membranes. It starts at
the birth of the baby and ends with the complete expulsion of the
placenta and the membranes.
4. Fourth stage is the stage of permanent retraction of the uterus also
known as the golden hour and strict monitoring of maternal vitals,

25. PHASE 3 – ACTIVE LABOR
• 3Ps needed for normal labor
Power (contractions) Passage (pelvis)
Passenger(baby)
• UTERINE CONTRACTIONS – During labor, the contractions are painful and
lead to dilation of the cervix.
• Uterine contractions start at the cornua (Pacemaker)
• Contractions are predominant over the fundus (Fundal predominance)
• Contractions spread throughout the uterus at 2cm/sec, depolarizing the
whole organ within 15 sec

26. PHASE 3 – ACTIVE LABOR
• PASSAGE – (BIRTHCANAL)
• Polarity of uterus – Isthmus in non-pregnant female is 0.5 cm.
• During pregnancy, isthmus enlarges to form lower segment
• The length of lower uterine segment at labor – 10 cm
• During labor – The upper uterine segment actively contracts, lower
segment dilates passively called as polarity of uterus, allowing the baby to
move out of the uterus.
*RETRACTION OF UTERINE MUSCLES
• During labor, when the upper uterine segment contracts and relaxes.
The muscle will not come back to its original length. The muscle gets
shortened as the contraction progresses. This is called RETRACTION.

27. CONTRACTION & RELAXATION VS RETRACTION

28. PHASE 3 ACTIVE LABOR –
EVENTS IN FIRST STAGE OF LABOR –
STAGE OF CERVICAL EFFACEMENT AND
DILATATION
• Concerned with the preparation of the birth canal to facilitate
expulsion of the fetus in the second stage.
• Main events that occur are :-
• A) Dilatation and effacement of the cervix
• B) Full formation of lower uterine segment
• EFFACEMENT OR TAKING UP OF CERVIX: It is the process by which
the muscular fibers of the cervix are pulled upward and merges with
the fibers of the lower uterine segment.
• In primigravidae, effacement precedes dilatation of the cervix.
• In multiparae, both occur simultaneously.

30. PHASE 3 ACTIVE LABOR –
EVENTS IN FIRST STAGE OF LABOR –
STAGE OF CERVICAL EFFACEMENT AND
DILATATION
• LOWER UTERINE SEGMENT –
• During labor the demarcation of an active upper segment and a
relatively passive lower segment is more pronounced.
• The wall of the upper segment becomes progressively thickened with
thinning of lower segment.
• Timing – Pronounced in late first stage and attains its maximum in
second stage.
• A distinct ridge is produced at the junction of the two called
PHYSIOLOGICAL RETRACTION RING.

33. PHASE 3 ACTIVE LABOR –
EVENTS IN SECOND STAGE OF LABOR –
STAGE OF FETAL EXPULSION
• This stage is concerned with the descent and delivery of the
fetus through the birth canal.
• Second stage has two phases:
• PROPULSIVE- From full dilatation until head touches the pelvic floor
• EXPULSIVE – Mother has irresistible desire to ‘bear down’ and push until
the baby is delivered.
• Expulsive force of uterine contraction and retraction is aided by
voluntary contraction of the abdominal muscles called ‘bearing
down efforts’.

34. PHASE 3 ACTIVE LABOR –
EVENTS IN THIRD STAGE OF LABOR –
STAGE OF PLACENTAL SEPARATION AND
EXPULSION
• It comprises the phase of placental separation, its descent to
the lower segment and finally its expulsion with the
membranes.
• Marked retraction of uterus after fetus expulsion, reduces the
surface area at the placental site.
• As the placenta is inelastic, a shearing force is instituted
between the placenta and the placental site, leading to its
separation.
• Plane of separation runs through deep spongy layer of decidua

36. PHASE 3 ACTIVE LABOR –
EVENTS IN THIRD STAGE OF LABOR –
STAGE OF PLACENTAL SEPARATION AND
EXPULSION
• There are two ways of separation of placenta:
• A) CENTRAL SEPARATION (SCHULTZE) – Detachment of placenta
from its uterine attachment starts at the center resulting in
accumulation of blood behind the placenta (retroplacental
hematoma).
• B) MARGINAL SEPARATION (MATHEWS-DUNCAN) – Separation
starts at the margin, as it is mostly unsupported. It is found
more frequently.

37. PHASE 3 ACTIVE LABOR –
EVENTS IN THIRD STAGE OF LABOR –
STAGE OF PLACENTAL SEPARATION AND
EXPULSION

38. PUERPERIUM (INVOLUTION)
• It is a stage of myometrial retraction and cervical repair.
• Puerperium is the period during which the pregnancy induced
maternal anatomical and physiological changes return to the
nonpregnant state.
• This period of change lasts for 6 weeks.
• Organs, particularly the reproductive organs return to the non-
pregnant or near normal state except for the lactating breasts,
which remain active throughout this period.

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