What is Active Labor?
Active labor is the phase of the first stage of labor characterized by an accelerated rate of cervical dilation that transitions from the relatively flat slope of the latent phase to a more rapid progression, continuing until complete cervical dilation (10 cm) is achieved. 1
Defining Characteristics
The onset of active labor is identified by an increase in the rate of cervical dilation, not by reaching a specific centimeter measurement. 1 This acceleration can occur at various degrees of cervical dilation and requires serial vaginal examinations at least every 2 hours to detect when the dilatation rate increases from the negligible slope of latent phase to the more rapid active phase progression. 1
Key Diagnostic Features
The cervical dilation pattern is the only reliable indicator for prospectively identifying active labor onset, as uterine contractions inconsistently increase in intensity, frequency, and duration and do not reliably distinguish when active phase begins. 1
No diagnostic manifestations demarcate the onset other than the accelerating dilatation itself—there is no abrupt change in contraction characteristics that signals the transition. 1
Montevideo Units and contraction palpation have limited value in determining active phase status, as contractile patterns have yielded little to facilitate differentiating normal from abnormal labor. 1
Normal Progression Rates
Active labor progresses at minimum rates of ≥1.2 cm/hour for nulliparous women and ≥1.5 cm/hour for multiparous women. 1, 2 These represent the fifth centile lower limits, meaning rates slower than these thresholds indicate protracted active phase labor. 1
Duration and Endpoints
Active labor ends with complete cervical dilation (10 cm), often preceded by a brief deceleration phase that is usually short in duration and frequently undetected. 1
Most cervical dilation occurs during the active phase, making accurate identification of its onset critical for appropriate labor management. 1
Common Pitfalls in Identification
A critical error is assuming active labor begins at a fixed cervical dilation measurement (such as 6 cm). 3 While some guidelines reference specific centimeter thresholds, research demonstrates that many nulliparous women are already in active phase before reaching 6 cm, while others remain in latent phase even after 6 cm. 3, 4
Less than 50% of normal labors become active by 4 cm dilation, and by 5 cm, approximately 74% are active (89% when excluding dystocia cases). 4
The transition point varies widely among individuals, occurring across a range of cervical dilations when the rate change from latent to active phase is detected. 1
Failing to perform serial examinations every 2 hours prevents accurate detection of the rate acceleration that defines active labor onset. 1
Clinical Management Implications
Identifying active labor accurately is essential because major labor abnormalities can only be diagnosed during the active phase, not during the latent phase which is devoid of major labor disorders except for prolonged duration. 1
Abnormal Patterns Detectable in Active Labor
Protracted dilatation (rate below minimum thresholds), arrest of dilatation, prolonged deceleration phase, and failure of descent are aberrant patterns identifiable during active labor. 1
Underlying factors include cephalopelvic disproportion (occurring in 25-30% of protracted active phase cases), excessive neuraxial block, poor uterine contractility, fetal malpositions, malpresentations, uterine infection, maternal obesity, advanced maternal age, and previous cesarean delivery. 1, 2
When active-phase disorders are identified, cesarean delivery is justifiable if there is compelling clinical evidence of disproportion, particularly with prolonged deceleration phase which is strongly associated with disproportion and second stage abnormalities. 1
Practical Assessment Approach
Graph cervical dilation serially against time using a partogram to visualize the rate of progression and identify the inflection point where the curve transitions from flat (latent) to steep (active). 1, 2 This graphical representation allows prospective identification of both the onset of active labor and any deviations from normal progression rates. 1