Mechanism of labor

1. MECHANISM OF LABOR IN
DIFFERENT TYPES OF
CEPHALIC PRESENTATIONS

2. Stages of Labor
• The first stage. It starts from the onset of labor pain
and ends with full dilatation of the cervix. It is in
other words the ―cervical stage of labor (dilation of
the cervix).Its average duration is about 12 hours. In
multiparae the duration of this stage may be shorter
(6-8 hours).
• The second stage. It starts with a full dilatation of
the cervix (not from the rupture of the membranes)
and ends with expulsion of the fetus from the birth
canal. Its average duration is 2 hours in
primigravidae and 30 minutes in multiparae. It is in
other words the stage of fetus‘ expulsion.

3. Stages of Labor
• The third stage. It begins after expulsion of the
fetus and ends with expulsion of the placenta
and membranes (afterbirth). Its average
duration is about 30 minutes. The duration,
however, is reduced to 5 minutes at active
management. It is in other words the stage of
afterbirth expulsion.

4. The first stage of labor may be divided into 3 phases
depending on the character of pain: latent, active and
slowing down phases.
• Latent phase. During the latent phase, the
uterine contractions are typically infrequent,
somewhat uncomfortable, and, in some cases,
not very strong, but they generate sufficient
force to cause slow dilation and some effacement
of the cervix. Latent phase of labor begins from
the onset of regular labor pain to 4 cm opening
of the cervix, the duration of which is about 5
hours in multiparae and 6.5 hours in
primigravidae. The rate of the cervix dilatation is
about 0.35 cm/hour.

5. • Active phase. This phase follows the latent phase and is
characterized by a progressive cervical dilation,
progressive labor pains. The duration is about 1.5 — 3
hours. It takes place from 4 to 8 cm opening of the
cervix. The rate of the opening is about 1.5 — 2 cm/h in
multiparae, 1-1.5 cm/h in nulliparae.
• Slowing down phase. This phase follows the second
phase and is characterized by some decreasing of pain
intensity. It begins soon after the 8 cm cervix opening till
the full opening of the cervix. The labor pains become
less strong and intensive; the intervals between the pains
are rather short. For example, the duration of pain is
about 45–40 seconds, and intervals are about 2 — 2.5
minutes. The duration of this phase is about 1–2 hours:
the rate of the cervix opening is 1-1.5 cm/h.

6. Mechanism of Labor in Occipito-
Anterior Presentation
• It is the most typical, normal type of labor.
Occipito-anterior presentation means that the lie
of the fetus is longitudinal, position is left or
right, type of position is anterior, i.e. fetal back is
to anterior wall of the uterus. Broadly speaking,
the mechanism of labor is as follows: in passage
the head (and the trunk) makes three
movements — engagement or entry into the
pelvis, rotation or adaptation to the shape of the
pelvis, and disengagement or exit from the
pelvis.

7. Mechanism of Labor
• In normal labor, the head enters the pelvic brim
more commonly through the available transverse
diameter (70%) and to a lesser extent through one of
the oblique diameters. Thus, transverse diameter of
the inlet becomes the diameter of engagement. In
normal labor, the sagittal suture often strictly
corresponds to the available transverse diameter of
the inlet. It is called synclitism. But sometimes there
may be physiologic short-term asynclitism, when the
sagittal suture does not strictly correspond to the
transverse diameter of the inlet. Instead, it is either
deflected anteriorly towards the symphysis pubis or
posteriorly towards the sacral promontory.

8. Mechanism of Labor
• When the sagittal suture lies anteriorly, the posterior
parietal bone becomes the leading presenting part and is
called posterior parietal presentation (Litzmann‘s
asynclitism, or posterior asynclitism). This is more
frequently found in primigravidae because of good
uterine tone and a tight abdominal wall. If the sagittal
suture lies more posteriorly with the result that the
anterior parietal bone becomes the leading presenting
part, it is called anterior parietal presentation (anterior
asynclitism, or Negele‘s asynclitism). It is more
commonly found in multiparae. Short-term asynclitism
may occur in normal labor: the posterior parietal bone
hangs over the inlet with the sagittal suture directed
downwards and forwards.

9. • In time the molding (configuration) of the head happens, and the
anterior parietal bone descends behind the symphysis in a
downward and backward direction following a curved axis of
descent. The long–term asynclitism usually occurs in pathological
labor, for example, in labor with contracted pelvis

10. The principal moments of mechanism of normal labor are
as follows:
• The 1st moment is flexion of the head. When
the descending head meets with resistance of
both cervix and walls of the pelvis, flexion of the
fetal head normally occurs.

11. • The chin is brought into close contact with the
fetal breast. The mechanical gain in flexion is
that instead of an occipitofrontal diameter of 11
cm and a circumference of 35 cm, the
suboccipitobregmatic extent with a diameter of
9–9.5 cm and a circumference of 31 cm is
presented to the birth canal

12. • The 2nd moment is internal rotation of the head.
It is a movement of great importance without which
there will be no further descent. This movement is
not accomplished until the head has reached the
level of the ischial spines (the 3rd pelvic plane —
obstetrical outlet). The head descends into the birth
canal and rotates simultaneously on its longitudinal
a xis so that the occiput (the posterior fontanelle)
from its original position turns anteriorly (towards
the symphysis pubis), while the sinciput (the
anterior fontanelle) rotates posteriorly (towards the
sacrum).
Mechanism of Labor

13. Mechanism of Labor
• At the beginning of this movement the sagittal
suture is aligned with one of the oblique diameters.
At the end of this movement (at the outlet of the
pelvis) the sagittal suture of the head is parallel to
the anteroposterior diameter of this plane of pelvis,
the leading point (posterior fontanelle) is turned to
the symphysis pubis. Such a position of the head
means that the internal rotation of the head is
completed. The internal rotation of the head with
small fontanelle turned anteriorly is named correct
internal rotation of the head.

14. Mechanism of Labor
• After internal rotation of the head, the further
descent occurs until the subocciputal fossa lies
underneath the pubic arch.

15. Mechanism of Labor
• The 3rd moment is extension of the head. When a
strongly flexed head reaches the pelvic outlet, it meets
with resistance of the pelvic floor muscles. Contractions
of the uterine and abdominal muscles push the fetus in
the direction of the sacral apex and coccyx. The muscles
of the pelvic floor oppose that thrust of the fetal head in
this direction and thus deflect its movement anteriorly
towards the pudendal cleft. The resultant force causes
the head to deflex when the posterior cranial fossa
passes beyond the inferior margin of symphysis which
acts as a fulcrum. The point of contact of subocciputal
fossa and pubic arch is called the point of fixation (or
hypomochlyon).

16. • The head revolves around the point of fixation
(extension of the head), so the sinciput, the face
and the chin gradually appear through the vulva.
Thus the head is delivered. So, the extension of
the head occurs around the point of fixation

17. Mechanism of Labor
• The 4th moment is external rotation of the head. It is a rotation
movement of the head visible externally due to internal rotation of
the shoulders. In this movement, the occiput returns to the oblique
position from which it started and then to the transverse position.
This movement corresponds to the rotation of the fetal body,
bringing the shoulders into an anteroposterior diameter of the
pelvic outlet

18. Mechanism of Labor
• The 5th moment is delivery of shoulders and trunk. After the
shoulders are positioned in anteroposterior diameter of the outlet,
further descent takes place until the anterior shoulder is fixed below
the symphysis pubis. By a movement of lateral flexion of the spine
the posterior shoulder rolls up over the perineum, after which the
anterior shoulder comes from behind the pubis. The rest of the
trunk is then expelled out by lateral flexion

19. Mechanism of Labor in
Occipito-Posterior Presentation
• Occipito-posterior positions are encountered in
about 25% of all vertex presentations. In this case
the lie of the fetus is longitudinal, attitude of the
fetus is flexion, and the position is left or right, but
type of position is posterior. Thus, one can find a
small fontanelle towards the maternal sacrum by
vaginal examination. During the second stage of
labor the great majority of posterior types of
position become converted into anterior by the
forward rotation (by 135 degrees) of the occiput.
This is the normal mechanism which occurs in about
80-90% of occipito–posterior cases.

20. • In the remainder the mechanism of labor is
posterior, and the duration of labor is longer
than that in occipito-anterior presentation, but it
is a type of physiological mechanism.

21. Mechanism of Labor
• The 1st moment is flexion of the head. The head
is flexed and begins to engage to the inlet with
its suboccipito-frontal diameter (10-10.5 cm),
while the sagittal suture is on the transverse
diameter of the inlet. Denominator is a midpoint
between a big and small fontanelle. There may
be short-term asynclitism because of the lateral
inclination of the head (sacral rotation).

22. Mechanism of Labor
• The 2nd moment is internal rotation of the
occiput posteriorly (small fontanelle towards the
sacral promontory). After the internal rotation of
the head which usually takes place with the head
occupying the 3rd, may be the 4th plane of the
pelvis, descending of the head continues. Then
the crowning of the head occurs. The first point
born is denominator; then the surrounding areas
are born. When the point of fixation is born, the
3 rd moment of labor starts.

23. Mechanism of Labor
• The 3rd moment is flexion of the head which
occurs after the anterior margin of the haired
part of the forehead lies underneath the pubic
arch (it is the first point of fixation). The head
begins to revolve around the point of fixation (so
flexion of the head begins). The successive parts
of the fetal head to be born through the
stretched vulval outlet are forehead (brow),
vertex, posterior (small) fontanelle.

24. Mechanism of Labor
• The 4th moment is extension of the head.
During this moment the subocciputal fossa lies
in direct contact with the anterior margin of the
perineum. The subocciputal fossa is the second
point of fixation; then the head begins to revolve
around this point of fixation and the head
extension occurs. The successive parts to be born
are: face and chin. Thus the delivery of the head
is completed.

25. Mechanism of Labor
• The 5th moment is external rotation of the head
and internal rotation of the shoulders.
• The 6th moment is delivery of the shoulders and
trunk by lateral flexion.

26. Thank you for your attention