MECHANISM OF LABOR

•	Physiology.
•	Anatomy.
•	Stages.
•	Mechanism.

The Physiology of Labor

1.	Onset and initiation of labor:

a.	Corticotrophin-releasing factor (CRF) may be the factor responsible for initiating labor.

b.	Role of prostaglandins:

i.	stimulating contractions.

ii.	ripening and increasing the sensitivity of the cervix to oxytocin.

c.	Mechanism: ripening and increasing the sensitivity of the cervix to oxytocin.

d.	Cervical ripening:

i.	results in structural changes to the collagen and connective tissue matrix.

ii.	occurs by the destruction of collagen fibers.

e.	Formation of myometrial gap junctions:

i.	these enable the myometrial fibers to contract effectively as a single un it.

ii.	prostaglandins stimulate their formation while beta-adrenergic compounds inhibit them. 

f.	Uterine retractions:

i.	this is the progressive shortening of the uterine smooth muscle cells in the upper portion of the uterus as labor progresses.

ii.	after the cells contract they relax but they do not return to their original length.

iii.	the result is the development of the thicker, active, contracting segment in the upper portion of the uterus.

iv.	at the same time the lower segment of the uterus becomes thinner and more stretched.

v.	eventually the cervix is taken up into the lower segment of the uterus and forming a continuum with the lower uterine segment. 

g.	Uterine contractions:

i.	involuntary in nature.

ii.	they occur at intervals of 2 to 4 minutes.

iii.	duration varies from 30 to 60 seconds or longer.

iv.	intensity of intrauterine pressure generated with each contraction averages between 30 and 60mmHg. 

Anatomy of birth canal

1.	The birth canal is made up of bones of the true pelvis and the ligaments which bind them together with the associated muscular and fascial supports.

2.	The canal is curved gently from behind forward.

3.	Widest diameter at:

a.	Brim: transverse.

b.	Mid-pelvis: oblique.

c.	Outlet: antero-posterior.

4.	This is why the head tends to lie in a transverse position at the brim, an oblique position in the mid-pelvis, and an antero-posterior position at the outlet. 

Stages of Labor

1.	First stage:

a.	Onset of labor to full dilatation of cervix (10cm).

b.	Duration:

i.	primip: 6 – 18 hours (8 hours latent).

ii.	multip: 2 – 10 hours (6 hours latent).

2.	Second stage:

a.	Full dilatation to delivery of the fetus.

b.	Duration:

i.	primip: 30 minutes to 3 hours.

ii.	multip: 5 minutes to 30 minutes.

3.	Third stage:

a.	Delivery of the fetus to delivery of the placenta.

b.	Duration: 5 to 30 minutes. 

Definitions

1.	Lie:

a.	Long axis of fetus with respect to the long axis of the uterus.

b.	Longitudinal, transverse and oblique.

2.	Attitude:

a.	Relation of the fetal head and limbs to the trunk.

b.	Flexion and extension.

3.	Presentation:

a.	Part of fetus which is lowermost in the uterus and presents on vaginal examination.

b.	Cephalic, breech and shoulder.

4.	Engagement:

a.	Occurs when the longest diameter of the presenting portion has passed through the pelvic inlet.

b.	Assessed by the amount of head palpable above inlet in fifths.

c.	Equal or less than 2/5: engaged head.

5.	Station:

a.	Relationship of the leading presenting part to the maternal ischial spine.

b.	Graded from – 2 to 2. 

6.	Denominator: an identifiable bony landmark chosen for each presentation of the fetus; the occiput is the denominator for vertex presentation.

7.	Position:

a.	Relationship of the denominator of the presenting part to the various quadrants in the maternal pelvis.

b.	Cephalic: fetal occiput.

c.	Breech: sacrum.

8.	Crowning: when the largest diameter of the head has passed through the pelvis outlet, associated with irreversible stretching of perineum.

Mechanism of Labor

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

2.	Descent:

a.	Occurs because of the contraction of the uterus which push the fetus down and at the same time stretch and draw up the muscle fibers of the lower uterine segment.

b.	This process is continuous throughout labor although, as with cervical dilatation, there is a phase in the second half of the first stage, when descent is more rapid.

c.	Often, the fetal head has already descended into the pelvis and become engaged before the onset of labor in nulliparous mothers.

3.	Engagement.

4.	Flexion:

a.	As the head descends into the narrower mid-cavity, flexion occurs resulting in a suboccipito-bregmatic diameter.

b.	Probably as a passive movement in part due to the surrounding structures.

5.	Internal rotation:

a.	Rotation of the head inside the pelvis from LOT (left occipito-transverse) to LOA (left occipito-anterior) and finally the direct OA (occipito-anterior) position.

b.	Due to the anatomy of the lower pelvis and the resistance of the pelvic floor. 

6.	Extension:

a.	Following completion of internal rotation the occiput is underneath the symphysis pubis and the bregma is near the lower border of the sacrum.

b.	The well-flexed head now extends, with the occiput escaping from underneath the symphysis pubis and starting to distend the vulva; crowning of the head.

c.	The head extends further and with the symphysis pubis as the fulcrum point, the bregma, face and the chin appear in succession.

d.	This extension and movement minimizes soft tissue trauma by utilizing the smallest diameters of the head for the birth. 

7.	Restitution:

a.	As the fetal head reaches the pelvis floor, the shoulders remain in the oblique diameter.

b.	As the occiput escapes from the vulva, the head aligns itself with the shoulders, rotating from OA to LOA.

8.	External rotation:

a.	In order to be delivered, the shoulders have to rotate into the direct occipito-anterior position.

b.	When this occurs, the occiput rotates from LOA to OT.

c.	The anterior shoulder is now under the symphysis pubis and delivers first, and the posterior shoulder delivers subsequently.