The mechanism of normal labour By Dr. sallama kamel بسم الله الرحمن الرحيم The mechanism of normal labour By Dr. sallama kamel

Before discussing the mechanism of labour we should describe the following terms The lie: This is the relation which the fetus bears to the longitudinal axis of the uterus. The lie may be 1.Longitudinal. 2.Oblique. 3.Transverse. Presentation: The presenting part of the fetus is that part which is in or over the pelvic brim and in relation to the cervix.

-When the head occupies the lower segment of the uterus ,the presentation is termed cephalic (in 96% of pregnancies). -If the head is well flexed on the trunk the vertex (the area between the two parietal bones, anterior and posterior fontanelles ) will present. -If the head is fully extended on the spine there will be face presentation. -If it is partly extended there will be brow presentation

*If the buttocks occupies the lower segment the presentation termed podalic or breech presentation. *If the fetus lies obliquely , the shoulder lies over the cervix and this is called shoulder presentation. *Any presentation other than a vertex presentation is described as a malpresentation.

*With breech presentation it is the sacrum Position: *This is the relationship between some selected part of the fetal presenting part (the denominator) to the maternal pelvis. *The denominator varies according to the presentation. *With vertex presentation the denominator is the occiput *With face presentation it is the chin (mentum) *With breech presentation it is the sacrum

Four positions are described for each presentation: For example, with vertex presentation, the occiput could be related to: 1.Left ilio-pectineal eminence → left occipito-anterior position (LOA). 2.Right ilio-pectineal eminence→ right occipito- anterior position (ROA). 3.Right sacroiliac joint→ right occipito-posterior position (ROP). 4.Left sacroiliac joint → left occipito-posterior position (LOP).

However ,during late pregnancy and during the first stage of labour the occiput most commonly lies in the transverse diameter of the pelvic brim. So the position either: -left occipito-transverse position (LOT) or -right occipito-transverse position (ROT)

Attitude (flexion or extension): -This term refers to the relation of the different parts of the fetus to one another. -Normally the head, back and limbs of the fetus are flexed. -In some abnormal presentations, the head or limbs may be extended.

In 96% of cases at term the fetus lies longitudinally with the head presenting . -The reason for this is that the fetus adapts itself by it’s movement to the shape of the uterus. -In the early months of pregnancy the amniotic fluid is more abundant, and the fetus can move freely, -but as pregnancy advances the fetus rapidly increase in size and the volume of the fluid becomes comparatively less, -so that the fetus is constrained to fit the shape of the uterus.

-When the attitude is one of complete flexion, the buttocks together with the adjacent parts of the thighs and the feet, constitute a mass which is larger than the head. -The cavity of the uterus at term is pear-shaped, with the wider end uppermost; therefore the fetus fits into it best when the breech lies in the upper part of the uterus and the head in the lower part.

Mechanism of labour with vertex presentation: The term ‘mechanism’ refers to the series of changes in position and attitude, which the fetus undergoes during it’s passage through the birth canal. -The relation of the fetal head and body to the maternal pelvis changes as the fetus descend through the pelvis. -This is essential so that the optimal diameters of the fetal skull are present at each stage of the descent.

The mechanism of labour is divided into 1.Engagement. 2.Descent. 3.Flexion. 4.Internal rotation. 5.Extension. 6.Restitution. 7.External rotation. 8.Delivery of the fetal body.

Engagement -The head normally enter the pelvis in the transverse position. -The anterior parietal bone slides past the symphysis pubis followed by the posterior parietal bone. -So the sagittal suture stays evenly oriented between the sacrum and the symphysis. Engagement occurred when the widest part of the presenting part has passed successfully through the pelvic brim or inlet.

*The widest part of the fetal head is the biparietal diameter. *The number of fifths of the fetal head palpable abdominally is used to describe whether engagement has take place or not. *If more than two-fifths of the fetal head is palpable abdominally then the head is not engaged. *Engagement occur in the vast majority of nulliparous women prior to labour.

Descent *Descent of the fetal head is needed before the further series of changes of flexion, internal rotation and extension. *During the 1st stage and the first phase of the 2nd stage of labour, descent of the fetus is secondary to uterine action. *In the second phase of the second stage of labour, descent of the fetus is helped by voluntary use of abdominal musculature.

Flexion: *The head is often well flexed before labour starts. *if flexion is incomplete when labour starts it become complete as the uterine contractions drive the head down into the lower uterine segment. This is because: 1.Any ovoid body being pressed through a tube tends to adapt it’s long diameter to the long axis of the tube. 2.The so called head lever, the occipito-spinal joint is nearer to the occiput than to the sinciput (forehead), so the head can be regarded as a lever with a long anterior and a short posterior arm.

*When the breech is pressed on by the uterine fundus, the fetus is subjected to axial pressure and the lever come into play. *The long anterior arm meets with more resistance than the short posterior arm and the head undergoes flexion.

*Flexion has the advantage of bringing the shortest sub-occipito-Bregmatic diameter(9.5cm) of the head into engagement, *So the posterior fontanelle of the skull will be felt at a lower level than the anterior fontanelle

Internal rotation: *The anatomy of the lower pelvis and the resistance of the pelvic floor predispose the head to arrive at the pelvic outlet in the anterior-posterior position. *And usually to an occipito-anterior rather than occipito-posterior position. *This internal rotation occurs because with a well- flexed head the occiput is leading and meets the sloping gutter of the levator ani muscles before the sinciput, which by their shape direct it anteriorly (any part hits the levator ani muscle first will rotate anteriorly).

Extension: *Following completion of internal rotation the occiput lies under-neath the symphysis pubis and the bregma (anterior fontanelle) is near the lower border of the sacrum. *The well flexed head now extends, with the occiput escaping from underneath the symphysis pubis and starting to distend the vulva this is known as the crowning of the head . the sub-occipito-frontal diameter is the escaping diameter (10 cm). *The head extend further and the forehead, face and chin appear in succession over the posterior vaginal opening and perineal body.

Restitution -When the head is delivering, the occiput is directly anterior. -As soon as it escapes from the vulva, the head aligns itself with the shoulders , which have entered the pelvis in the oblique position. -The slight rotation of the occiput through one-eighth of a circle is called restitution.

External rotation: -In order to be delivered, the shoulders have to rotate into direct anterior-posterior plane. -When this occurs , the occiput rotates through a further one-eight of a circle to the transverse position, this is called external rotation. -So the shoulders will be in the anterior-posterior position. -The anterior and right shoulder is under the symphysis pubis and delivered first. -The posterior and left shoulder delivered subsequently.

Delivery of the body: -The rest of the body is usually delivered without any difficulty.

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