The Mechanism of Labor Trivia Quiz

Answer: Vertex presentation

If the head of the baby enters the birth canal first, it is called cephalic presentation. Among those, if the vertex of head (a point on the back of the head) of the fetus presents (enters the birth canal) first, then it called a vertex presentation. This is the most common form of presentation of a fetus and usually a normal vaginal delivery is conducted for this. In a cephalic presentation, the fetus is said to be in longitudinal lie. Most other presentations, like the shoulder presentation and breech presentation, are abnormal and may need intervention by an obstetrician.

The major factors involved in the mechanism of labor are the pelvis and the soft parts, the fetus and the uterine forces.

Answer: Engagement

In women who are going through childbirth for the first time, engagement of the fetus may occur during the last few weeks of pregnancy but in those with previous births, it may occur only during the commencement of labor.

In a normal vertex presentation, the sagittal suture (a joint between the occipital bones of the skull) is the part that engages first. It usually lies perpendicular to the transverse axis of the pelvic inlet. This is called synclitism.

Sometimes the suture is not perpendicular nor parallel to the transverse axis of the inlet and it may be called as posterior or anterior asynclitism causing a change in the presentation and engagement of the fetus.

Answer: Vaginal mucosal changes

Descent usually occurs simultaneously with engagement. Factors that come into play during descent are the intrauterine fluid pressure and contractions, bearing down of certain muscles and the straightening of the fetal head. For descent to occur, the neck of the fetus bends sideways inside the pelvis and the parietal bone in the fetal skull slips behind the pubic symphysis of the mother's pelvis.

Answer: Flexion

Flexion results as soon as the descending head meets with resistance, from either the pelvic side wall or the pelvic floor. The chin gets into more intimate contact with the thorax and thus the suboccipito-bregamtic diameter (distance between a point below the occiput and the bregma of the fetal skull) engages with the pelvis.

Answer: True

This usually only occurs when the head of the fetus in a normal vertex presentation has reached the spines on the ischium of pelvic bone. This movement is usually related to the tonic resistance of the levator ani muscle. Other factors that influence internal rotation are the forward incline of the walls of the pelvis, the impetus given by the ischial spines and uterine contractions.

Answer: Extension

It happens after internal rotation. The main force that helps this movement is the powerful uterine contractions. It is in this stage that perineal tear may occur if the head descends too rapidly. Another factor that results in extension is the elastic resistance of the pelvic floor from below.

Answer: Restitution

As soon as the head is free outside the outlet, it rotates 45 degrees and the neck untwists. The head is back to its normal position in relation to the body. Abnormality in this step of the mechanism of labor may cause damage to the fetus and subsequent deformities or disabilities.

Answer: External rotation

Restitution and external rotation occur in quick succession and sometimes may appear as one continuous movement. But careful observation shows that restitution occurs first. IF the back of the head (occiput) is on the left, then the head turns towards the mothers left until the face points directly towards the right thigh of the mother. If the occiput is on the right, then the reverse happens and the fetal head points towards the left thigh of the mother.

Answer: Expulsion

In this stage, the uterine contraction continue until the front of the fetal shoulders is under the pubic symphysis and the back of the shoulder is over the perineum. There is a slight slippage of the shoulders from under the symphysis. After the expulsion of the shoulder of the fetus, the rest of the fetal body slips down the pelvic cavity and is thus delivered.

Answer: True

A transverse lie can be caused by a contracted pelvis, uterine abnormalities, placenta previa, prematurity, hydramnios or multiple pregnancies.

Diagnosis is usually done by an abdominal examination. A hard mass will be felt in the side of the abdomen.

During uterine contractions, it will not be possible to do any version methods to change the lie of the baby, so it is recommended to remove the fetus via a cesarean section. But, before labor, it is possible to change the position of the fetus by external version.