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Phases of Parturition

Parturition, the bringing forth of young, encompasses all physiological processes involved in birthing: the prelude to (phase 0), the preparation for (phase 1), the process of (phase 2), and the recovery from (phase 3) childbirth.

Phase 0

Phase 1 Activation

Phase 2 Stimulation

Phase 3 Involution

Prelude to parturition

Preparation for labor

Processes of labor

Parturient recovery

Myometrial Changes

The uterine smooth muscle must undergo a series of changes during phase 1 to prepare for labor. During phase 1, there is a striking increase in myometrial oxytocin receptors. There are increased numbers and surface areas of myometrial cell gap junction proteins such as connexin-43. Together these changes result in increased uterine irritability and responsiveness to uterotonin.


Phase 2 of Parturition: The Process of Labor

Phase 2 is synonymous with active labor, that is, the uterine contractions that bring about progressive cervical dilatation and delivery.

The first stage is divided into a relatively flat latent phase and a rapidly progressive active phase. In the active phase, there are three identifiable component parts: an acceleration phase, a linear phase of maximum slope, and a deceleration phase.


Mechanical stretching of the cervix enhances uterine activity in several species, including humans. This phenomenon has been referred to as the Ferguson reflex.


Endothelins are very powerful inducers of myometrial smooth muscle contraction, and endothelin receptors are demonstrable in myometrial tissue. Enkephalinase catalyzes the degradation of endothelin-1.


The Key Factors Thought to Regulate the Phases of Parturition

Phase 0 (Quiescence )

Phase 1 (Activation)

Phase 2 (Stimulation)

Phase 3 (Involution)










Uterine stretch



Nitric oxide




Parts of Fetal Skull




Anterior fontanelle


Between bregma and root of nose


Between root of nose and supraorbital ridges and junction of the floor of the mouth with neck


Bony prominence behind lambda


Diamond prominence behind anterior and posterior fontanelles and parietal eminences


Fetal Skull: Molding

  1. Due to descent, the frontal bones slip under parietal bones, resulting in molding
  2. Parietal bones can also slip under each other or under occipital bone
  3. Molding reduces head circumference
  4. Degree of molding (assessed vaginally):
    1. 0: Suture lines separate
    2. +1: Suture lines meet
    3. +2: Suture lines overlap but are reduced
    4. +3: Suture lines overlap but are irreducible

Varieties of Cephalic Presentations in Different Attitudes


Attitude of Head


1. Suboccipitobregmatic-9.5 cm, extends from the nape of the neck to

 Complete flexion


the center of the bregma



2. Suboccipitofrontal-10 cm, extends from the nape of the neck to the

 Incomplete flexion


anterior end of the anterior fontanelle or center of the sinciput



3. Occipitofrontal-ll.5 cm, extends from the occipital eminence to the

 Marked deflexion


root of the nose (glabella)



4. Mentovertical-14 cm, extends from the midpoint of the chin to the

 Partial extension


highest point on the sagittal suture



5. Submentovertical-ll.5 cm, extends from junction of floor of the

 Incomplete extension


mouth and neck to the highest point on sagittal suture



6. Submentobregmatic-9.5 cm, extends from junction of floor of the

 Complete extension


mouth and neck to the center of the bregma



Important Diameters of Pelvis


Measurement (cm)

 True / anatomical conjugate


 Obstetric conjugate


 Diagonal conjugate


 Transverse (inlet)






 AP (cavity)


 Transverse (cavity)




 Posterior sagittal (obstetrical outlet)


 Posterior sagittal (anatomical outlet)




  1. The fetus enters the pelvis in the left occiput transverse (LOT) position in 40% of labors and in the right occiput transverse (ROT position in 20%.
  2. In about 20% of labors, the fetus enters the pelvis in an occiput posterior (OP) position.

Importance of Plane of Least Dimension

  1. Curve of carus bends forward
  2. Origin of levator ani begins
  3. Internal rotation occurs here
  4. Station is '0' at this plane
  5. Pudendal block given here

Stages of Labor




From the onset of true labor to full dilation of cervix


From full dilation of cervix to birth of the baby


From birth of the baby to delivery of the placenta


1h observation period following delivery of the placenta


Physiological chills are seen in the fourth stage of labor.

  1. The cardinal movements of labor are engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion.
  2. The mechanism by which the biparietal diameter, the greatest transverse diameter of the fetal head in occiput presentations, passes through the pelvic inlet is designated as engagement.


The movement is the first requisite for the birth of a newborn.

Descent is brought about by one or more of four forces:

  1. Pressure of the amniotic fluid
  2. Direct pressure of the fundus upon the breech with contractions
  3. Bearing down efforts of maternal abdominal muscles
  4. Extension and straightening of the fetal body

American College of Obstetricians and Gynecologists (ACOG) began using a classification of station that divides the pelvis above and below the spines into five parts. These divisions represent centimeters above and below the spines. Thus, as the presenting fetal part descends from the inlet toward the ischial spines, the designation is -5,-4, -3, -2, -1, and then 0 station (at the spine). Below the ischial spines, the presenting fetal part passes +1, +2, +3, +4, and +5 stations to delivery.


Labor course is divided functionally on the basis of dilatation and descent curves into:

  1. Preparatory division, including latent and acceleration phases
  2. Dilatational division, occupying the phase of maximum slope of dilatation
  3. Pelvic division, encompassing both deceleration phase and second stage concurrent with the phase of maximum slope of descent.


Although the differential diagnosis between false and true labor is difficult at times, it can usually be made on the basis of the contractions, as follows:

True Labor

False Labor

Contractions occur at regular intervals

Contractions occur at irregular intervals

Intervals gradually shorten

Intervals remain long

Intensity gradually increases

Intensity remains unchanged

Discomfort is in the back and abdomen

Discomfort is chiefly in the lower abdomen

Cervix dilates

Cervix does not dilate

Discomfort is not stopped by sedation

Discomfort usually is relieved by sedation

Pain Pathway of labour

In the early stages of labour pain is mainly uterine in origin because of painful uterine contraction


“The pain of uterine contractions is distributed along the cutaneous nerve distribution of T10 to L1

In later stages pain is due to dilatation of the cervix.


“The pain of cervical dilatation and stretching is refered to the back through sacral plexus.

Description: 1

Caldeyro-Barcia and Poseiro from Montevideo, Uruguay, were pioneers who have done much to elucidate the patterns of spontaneous uterine activity throughout pregnancy.

They also introduced the concept of Montevideo units to define uterine activity. By this definition, uterine performance is the product of the intensity (increased uterine pressure above baseline tone during contraction) in millimeters of mercury multiplied by contraction frequency per 10 min. For example, three contractions in 10 min, each of 50 mm Hg intensity, would equal 150 Montevideo units.

According to Caldeyro-Barcia and Poseiro, clinical labor usually commences when uterine activity reaches values between 80 and 120 Montevideo units. This translates into approximately three contractions of 40 mm Hg every 10 min.

Intrauterine pressure during labor







40-50 mmg hg

100-120 mmg-hg

100-120 mmg hg

Origin and Propagation of Contractions

The normal contractile wave of labor originates near the uterine end of one of the fallopian tubes; thus, these areas act as "pacemakers." The right pacemaker usually predominates over the left and starts the great majority of contractile waves. Contractions spread from the pacemaker area throughout the uterus at 2 cm/s, depolarizing the whole organ within 15 s. This depolarization wave propagates downward toward the cervix. Intensity is greatest in the fundus.

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