What is the most common cause of first stage labour dystocia?

Most Common Cause of First Stage Labour Dystocia

Hypotonic uterine contractions are the most common cause of first stage labour dystocia. 1

Understanding First Stage Labour Dystocia

First stage labour dystocia occurs during the active phase of labour, which begins when cervical dilation transitions from the relatively flat slope of the latent phase to a more rapid slope. The active phase can begin at various degrees of dilation, with no diagnostic manifestations demarcating its onset other than accelerating dilation. 1

Several aberrant labour patterns can be detected during the active phase:

• Protracted dilation
• Arrest of dilation
• Prolonged deceleration phase
• Failure of descent

Underlying Causes of Labour Dystocia

While several factors can contribute to first stage labour dystocia, poor uterine contractility (hypotonic uterine contractions) is the predominant cause. Other contributing factors include:

• Cephalopelvic disproportion (CPD)
• Excessive neuraxial block
• Fetal malpositions (especially occiput posterior position)
• Fetal malpresentations
• Uterine infection
• Maternal obesity
• Advanced maternal age
• Previous cesarean delivery 1

Pathophysiology of Hypotonic Uterine Contractions

Uterine contractions create the primary driving force that results in cervical dilation and propels the fetus through the birth canal. When these contractions are inadequate in strength, frequency, or coordination, labour progress is impaired. 1

The mechanisms responsible for the initiation and maintenance of adequate and synchronized uterine activity necessary for labour and delivery result from a complex interplay of hormonal, mechanical, and electrical factors. 2

Inadequate labour progress is a common challenge in intrapartum care, with labour dystocia being the most common indication for cesarean deliveries performed during labour. 2

Assessment of Uterine Contractility

Interestingly, assessing uterine contractions by palpation or using Montevideo Units has limited value in determining if a patient is in the active phase. This is because:

1. Contractions inconsistently increase in intensity, frequency, and duration throughout the first stage
2. No abrupt change in contraction characteristics occurs to distinguish when the active phase has begun
3. Contractile patterns have been extensively studied but have yielded little to facilitate differentiating normal from abnormal labour 1

Research has shown that the shape of uterine contractions can be predictive of labour progress. The fall-to-rise (F:R) ratio (duration of returning from contraction peak to baseline divided by duration of rise time from baseline to peak) is higher in women who lack labour progress. 3

Clinical Management Considerations

When hypotonic uterine contractions are identified as the cause of labour dystocia, management options include:

1. Oxytocin administration - carefully titrated based on uterine activity
2. Amniotomy (artificial rupture of membranes)
3. Intrauterine pressure catheter use in select cases 4

Important Cautions

Before oxytocin administration, evaluate:

• Pelvic dimensions and shape
• Potential cephalopelvic disproportion
• Fetal macrosomia
• Malposition or malpresentation
• Excessive molding without descent
• Maternal factors such as diabetes and obesity 5

Discontinue oxytocin immediately if:

• Uterine hyperactivity occurs
• Fetal distress is detected
• Evidence of CPD emerges 5

If CPD is suspected or cannot be ruled out, proceed to cesarean delivery rather than oxytocin augmentation. If no progress occurs within 2-4 hours despite adequate contractions, consider cesarean delivery. 5

Key Clinical Pearls

1. The dilatation pattern alone is reliable for prospectively identifying the onset of the active phase and the normality of its progression, provided it is being graphed serially in labour. 1

2. Hypotonic uterine contractions are often difficult to diagnose precisely, as the ability to distinguish between adequate and inadequate contractility has proved elusive. 1

3. The frequency of contractions is typically higher in women who progress to vaginal delivery compared to those requiring cesarean section due to lack of progress. 3

4. When an active-phase disorder is identified, cesarean delivery is justifiable if there is compelling clinical evidence of disproportion. 1