Describing the mechanismoflabour is a common topic for OSCEs and MCQs. Although on the surface it can appear complicated, breaking the process down into individual steps makes it much easier to understand.
Normal labour involves the widest diameter of the fetus successfully negotiating the widest diameter of the bony pelvis of the mother via the most efficient route.
The mechanism of labour covers the passive movement the fetus undergoes in order to negotiate through the maternal bony pelvis. Labour can be broken down into several key steps.
Key stages of labour
Extension of the presenting part
For the purposes of this guide, the fetal movements will be described in relation to a cephalic (vertex) presentation with a longitudinallie. This is a common (low risk) presentation.
To understand the mechanism of labour, you need some basic understanding of pelvicanatomy.
Borders of the pelvic inlet
Posteriorly: Sacral promontory
Laterally: Iliopectineal line
Anteriorly: Pubic symphysis
Borders of the pelvic outlet
Posteriorly: Tip of the coccyx
Laterally: Ischial tuberosity
Anteriorly: Pubic arch
Since the transversediameter is greaterthan the antero-posterior (AP) diameter in the pelvicinlet, the widest circumference of the fetalhead descends in a transverseposition. However, when it gets closer to the pelvicoutlet, the nature of the pelvic floor muscles encourages the fetal head to rotate from a transverseposition to an anterior-posterior position, as the APdiameter is greaterthan the transversediameter.
Fetal head diameter varies depending upon the degree of neck flexion
It is also important to know how the circumference of the fetal head varies with different degrees of neck flexion:
Suboccipitobregmatic (vertex, flexed) is 9.5cm
Occipitofrontal (vertex, neutral flexion) is 11.0cm
Submentobregmatic (face) is 9.5cm
Verticomental (brow) is 13.5cm
The fetus descends into the pelvis.
In the primigravida this is likely to occur from 38 weeks gestation onwards, in a multigravidawoman, this may not occur until labour is established.
Descent is encouraged by:
Increased abdominal muscle tone
Braxton hicks in the late stages of pregnancy
Fundal dominance of the uterine contractions during labour
Increased frequency and strength of contractions during labour
As the head descends, it moves towards the pelvicbrim in either the left or rightoccipito-transverse position (this means the occiput can be facing the left side or right side of the mother’s pelvis).
This is when the largestdiameter of the fetalheaddescends into the maternalpelvis.
The term engagement is referring to the widest part of the fetal head successfully negotiating its way down deep into the maternal pelvis. Engagement is identified by abdominal palpation, where the fetal head is 3/5th palpable or less.
As the fetus descends through the pelvis, fundal dominance of uterine contraction exerts pressure down the fetalspine towards the occiput, forcing the occiput to come into contact with the pelvicfloor. When this occurs the fetalneckflexes (chin to chest) allowing the circumference of the fetal head to reduce to sub-occipitobregmatic (9.5cm).
In this position, the fetalskull has a smallerdiameter which assists passage through the pelvis.
The pelvicfloor has a guttershape with a forward and downwardslope, encouraging the fetal head to rotate from the left or right occipito-transverse position a total of 90-degrees, to an occipital-anterior (occiput facing forward) position, to lie under the subpubic arch.
With each maternalcontraction, the fetalhead pushes down on the pelvicfloor. Following each contraction, a rebound effect supports a smalldegree of rotation. Regular contractions eventually lead to the fetal head completing the 90-degree turn.
This rotation will occur during establishedlabour and it is commonly completed by the startofthesecondstage. Further descent leads to the fetus moving into the vaginalcanal and eventually, with each contraction, the vertex becomes increasingly visible at the vulva.
When the widestdiameter of the fetalhead successfully negotiates through the narrowestpart of the maternalbonypelvis, the fetal head is considered to be ‘crowning’. This is clinically evident when the head, visible at the vulva, no longer retreats between contractions. Complete delivery of the head is now imminent and often the woman, who has been pushing, is encouraged to pant so that the head is born with control.
Extension of the presenting part
The occiput slips beneath the suprapubicarch allowing the head to extend. The fetal head is now born and will be facing the maternal back with its occiput anterior.
External rotation & restitution
Because the shoulders at the point of the head being delivered are only just reaching the pelvic floor they are often still negotiating the pelvic outlet and the fetus may naturally align its head with the shoulders. This is called restitution and visually you may see the headexternallyrotate to face the right or left medial thigh of the mother.
During the next contraction, the shoulders, having reached the pelvicfloor, will complete their rotation from a transverseposition to an anterior-posterior position. Evidence of this manoeuvre happening inside can be visualised by seeing the head externally rotating as the fetus keeps its spine aligned.
Delivery of the shoulders and body
Downwardtraction by the healthcare professional will assist the delivery of the anterior shoulder below the suprapubic arch.
This is followed by upward traction assisting the delivery of the posterior shoulder.
The fetal body will be delivered by the contractions, the health professional’s role is only to assist safe negotiation of this last stage.