Mechanism of normal labour
The mechanism of normal labour refers to a series of changes in position and attitude that the fetus undergoes during its passage through the birth canal. This process is essential so that the optimal diameters of the fetal skull are present at each stage of descent.
An understanding of the physiological and anatomical principles involved in labour is best summarized using the ‘3 Ps' which are the powers, the passages, and the passenger. The ‘powers' refers to forces: firstly, the contractions of the uterine muscle that result in propulsion of the fetus through the birth canal, and secondly, the maternal effort of pushing in the second stage of labour. The ‘passages' refers to the birth canal itself, which is made up of the bony pelvis, the muscles of the pelvic floor, and the soft tissues of the perineum. The ‘passenger' refers to the fetus in terms of its size, presentation, and position. When the 3 Ps are favourable, normal labour is likely to result in a spontaneous vaginal birth with minimal morbidity.Engagement
The fetal head normally enters the pelvis in a transverse position (more commonly to the left) taking advantage of the widest pelvic diameter. Engagement is said to have occurred when the widest part of the presenting part has passed successfully through the inlet. This occurs in the majority of nulliparous women prior to labour, usually by 37 weeks' gestation, and often later for multiparous women. The number of fifths of the fetal head palpable abdominally is used to describe whether engagement has taken place. If more than two-fifths of the fetal head is palpable abdominally, the head is not engaged.
Descent
Descent of the fetal head is needed before flexion, internal rotation, and extension can occur. During the first stage and passive phase of the second stage of labour, descent of the fetus occurs as a result of uterine contractions.
In the active phase of the second stage of labour, further descent of the fetus is assisted by voluntary efforts of the mother using her abdominal muscles and the Valsalva manoeuvre (‘active pushing').Flexion and internal rotation
The fetal head is not always completely flexed when it enters the pelvis. As the head descends into the narrower mid pelvis, flexion occurs. This passive movement occurs, in part, due to the surrounding structures and reduces the presenting diameter of the fetal head. If the head is well flexed, the occiput will be the leading point and on reaching the sloping gutter of the levator ani muscles, it will be encouraged to rotate anteriorly so that the sagittal suture now lies in the anteroposterior (AP) diameter of the pelvic outlet (i.e. the widest diameter).
Extension
Following completion of internal rotation, the occiput is beneath the symphysis pubis and the anterior fontanelle (bregma) is near the lower border of the sacrum. The well-flexed head now extends and the occiput escapes from underneath the symphysis pubis and distends the vulva. This is known as ‘crowning' of the head. The head extends further and the anterior fontanelle, face, and chin appear in succession over the posterior vaginal opening and perineal body.
Restitution and external rotation
When the head is delivering, the occiput is directly anterior. As soon as it crosses the perineum, the head aligns itself with the shoulders, which have entered the pelvis in an oblique position. This slight rotation of the occiput through one-eighth of the circle is called ‘restitution'. In order to be delivered, the shoulders have to rotate into the direct AP plane (widest diameter at the outlet). When this occurs, the occiput rotates through a further one-eighth of a circle to the transverse position. This is called external rotation.
Delivery of the shoulders and fetal body
When restitution and external rotation have occurred, the shoulders will be in the AP position at the pelvic outlet. The anterior shoulder is under the symphysis pubis and delivers first, and the posterior shoulder delivers subsequently. Normally the rest of the fetal body is delivered easily.