Definition, Stages and Mechanism of labor

Definition of labor, normal and abnormal labor, WHO criteria of normal labor, Stages of labor, mechanism of labor, factors affecting labor, duration of labor, hormonal factors during labor, formula to calculate estimate delivery date.

1. MECHANISM OF
LABOUR
Presentation by: Group H1

2. Series of events that take place in the
genital organs in an effort to expel the
viable products of conception (fetus,
placenta and the membranes) out of
the womb through the vagina into the
outer world is called labour.
LABOUR

3. The physiological and anatomical
principles involved in normal and
abnormal labor is best summarized
using the ‘3 Ps’, which are the
powers, the passages and the
passenger.
When 3Ps are favorable, normal
labour is likely to ensue, resulting in
an unassisted or spontaneous
vaginal birth.
NORMAL
LABOUR

4. In 1996, the World Health
Organization (WHO) defined normal
labour as follow:
▪It should be spontaneous in onset.
▪The infant is born spontaneously in
the vertex position between at term
through the birth canal.
▪After birth, mother and infant are in
good condition.
WHO
CRITERIA OF NORMAL LABOUR

5. When any of the 3Ps are unfavorable,
OR any deviation from the criteria of
normal labour then it is termed as
abnormal, resulting in the need for
intervention and with that, an increased
risk of morbidity or mortality.
ABNORMAL
LABOUR

6. The first important step is to
recognize when labor has started.
The Naegele’s formula is simple
arithmetic method for calculating the
EDD (estimated date of delivery)
based on the LMP (Last menstrual
period.)
Key points are Menstrual cycle of 28
days and gestational period of 280
days.
NAEGELE'S
FORMULA

7. There are two formulas:
First formula:
LMP date - 3 months from the LMP +
7 days in LMP + 1 year
Example:
LMP date: September 28, 2018- 3
months = June 28, 2018+7 days =
July 5, 2018+1 year = July 5, 2019
NAEGELE'S
FORMULA

8. Second formula:
LMP date + 7 days in LMP + 9 months
Example:
LMP date: September 28, 2018+7
days = October 5, 2018+ 9 months =
July 5, 2019
NAEGELE'S
FORMULA

9. The onset of labour occurs when the
factors that inhibit contractions and
maintain a closed cervix diminish and
are overtaken by the actions of factors
that do the opposite. These includes;
•Hormonal Factors
•Mechanical Factors
CAUSES OF ONSET
OF LABOUR

10. Stretching effect on the myometrium by
the growing fetus and liquor amnii.
Uterine stretch increases gap junction
proteins, receptors for oxytocin and
specific contraction associated
proteins.
MECHANICAL
FACTORS

11. •Estrogen: During pregnancy, most of the estrogens are
present in a binding form. During the last trimester, more free
estrogen appears increasing the excitability of the myometrium
and prostaglandins synthesis.
•Progesterone: Before labor, there is a drop in progesterone
synthesis leading to the predominance of the excitatory action
of estrogens.
•Oxytocin: Release from the posterior pituitary. It increases the
strength of uterine contractions. TheFerguson reflexis a
neuroendocrine reflex in which the fetal distension of the cervix
stimulates a series of neuroendocrine responses, leading to
oxytocin production
•Prostaglandins: E2 and F2α are powerful stimulators of
uterine muscle activity. It is synthesized by the chorion and the
decidua is enhanced, leading to an increase in calcium influx
into the myometrial cells.
HORMONAL
FACTORS

12. •This change in the hormones also increases gap
junction formation between individual myometrial
cells, which is necessary for coordinated uterine
activity.
•The production of corticotrophin-releasing
hormone (CRH) by the placenta increases in
concentration towards term and potentiates the
action of prostaglandins and oxytocin on myometrial
contractility.
•The fetal pituitary secretes oxytocin and the fetal
adrenal gland produces cortisol, which stimulates
the conversion of progesterone to estrogen.
HORMONAL
FACTORS

14. Labor is then divided into three
stages:
1.First stage: It begins with diagnosis of
the onset of labor and is complete
when full cervical dilatation has been
reached.
2.Second stage: It begins with full
cervical dilatation and ends with birth of
the baby.
3.Third stage: It begins with birth of the
baby and ends with complete delivery
of the placenta and membranes.
STAGES OF
LABOUR

15. The onset of labor is defined by strong,
regular, and painful contractions that cause
the cervix to change.
Labor is diagnosed with at least two
vaginal exams showing cervical progress.
When a woman feels contraction-like
pains, a doctor confirms labor through a
vaginal exam showing the cervix thinning
and dilating.
DIAGNOSIS
OF LABOUR

16. Retraction is a phenomenon of the uterus in labour in
which the muscle fibres are permanently shortened.

18. Time interval between the diagnosis of labour
to full dilatation of the cervix (10 cm).
Two phases:
Latent phase: (0 to 3-4cm cervical dilatation)
During this time, the cervix becomes ‘fully
effaced’. It usually lasts between 3 and 8
hours.
Active phase: (4 to 10cm cervical dilatation).
It usually lasting between 2 and 6 hours.
1
st
STAGE

19. Dilatation & Effacement of cervix

20. The time from full dilatation of the cervix to
delivery of the fetus or fetuses.
Two phases:
Passive phase describes the time between
full dilatation and the onset of involuntary
expulsive contractions. There is no maternal
urge to push and the fetal head is still
relatively high in the pelvis.
Active phase is distinguished by maternal
bearing down efforts and ends with delivery
of the baby.
Average duration is 2 hours.
2
nd
STAGE

21. It begins after expulsion of the fetus and
ends with expulsion of the placenta and
membranes (after-births).
Its average duration is about 15 minutes in
both primigravidae and multiparae.
A third stage lasting more than 30 minutes is
defined as abnormal.
3
rd
STAGE

22. There is no perfect labor length for all
women, but problems arise if labor is too fast
or too slow.
Morale drops after 6 hours in labor and
worsens after 12 hours.
Longer labor increases the risk of fetal
hypoxia and the need for operative delivery.
Prolonged labor is over 12 hours for first-
time mothers and over 8 hours for mothers
who have given birth before. Precipitous
labor is when the baby is born within 3 hours
of regular contractions starting.
DURATION OF
LABOUR

23. This refers to the series of changes in
position and attitude that the fetus
undergoes during its passage through the
birth canal.
MECHANISM
OF LABOUR

24. Presentation: It refers to the part of the
fetus’s body that enters the pelvic inlet.
Lie: Relation between the long axis of the
fetus to the long axis of the uterus.
Longitudinal Oblique Transverse
MECHANISM
OF LABOUR

25. Position: refers to presenting part of the fetus in relation to pubic
symphysis of mother.
Attitude: Degree of flexion and extension at upper cervical
spines.
MECHANISM
OF LABOUR

26. Engagement:
Engagement occurs when the
widest part of the head passes
through the pelvic inlet. For
primigravidae , this happens by 37
weeks of pregnancy. Engagement
is checked by feeling the fetal head
through the abdomen. If more than
two-fifths of the head can be felt, it
is not yet engaged.
The fetal head enters the pelvis in
a transverse position, using the
widest pelvic diameter.

27. Descent:
Here the baby descends from pelvic inlet
towards the pelvic floor. It occurs due to
uterine contractions (1
st
and first phase of 2
nd

stage) and abdominal muscle contraction
(second phase of 2
nd
stage).
Flexion:
The fetal head isn’t always fully flexed when
entering the pelvis. As it descends into the
narrower midpelvis, it flexes. This movement
helps reduce the head’s presenting diameter.

28. Internal Rotation:
If the head is well flexed, the occiput
leads and rotates anteriorly i.e. in OA
position, aligning with the pelvic
outlet’s widest diameter.
If the fetus starts to descend in the OP
position, it can rotate to the OA position,
which may lengthen labor.
And if the OP position persists, the
baby may be born “face to pubes,”
causing head extension and increasing
the diameter presented to the pelvic
outlet. This can lead to obstructed
labor, requiring instrumental delivery or
a C-section.

30. Extension:
After internal rotation, the occiput is under the pubic symphysis, and the
bregma is near the sacrum. The head extends, the occiput emerges from
under the pubic bone, and the vulva stretches, which is called
"crowning." As the head extends more, the face and chin appear.
Controlled extension reduces perineal trauma, but some tearing often
occurs in first births due to resistance from the perineum.

31. Restitution & External
Rotation
The head slightly rotate after
passing perineum to align with
shoulders. This spontaneous
realignment of the head with the
shoulders is called restitution.
The occiput have to further
rotate externally to a transverse
position. This is called External
rotation.

32. Delivery of shoulders
and fetal body:
Delivery of the shoulders
and body often involves
gentle downward traction
on the fetal head to
release the anterior
shoulder, followed by
upward traction to guide
the posterior shoulder
over the perineum,
facilitating the baby’s
delivery onto the
maternal abdomen.

37. REFERENCE