Stages of Active Labor
Active labor begins when the cervical dilation rate accelerates from the flat slope of latent phase—detected only through serial vaginal examinations at least every 2 hours—and progresses at ≥1.2 cm/hour in nulliparous women and ≥1.5 cm/hour in multiparous women. 1
Defining the Onset of Active Labor
- No specific centimeter threshold (such as 4 cm or 6 cm) reliably marks the start of active labor; the diagnosis depends solely on observing an accelerating dilation rate through serial examinations 1
- The transition from latent to active phase typically becomes evident around 6 cm dilation, though this varies by individual 2
- Contraction patterns are unreliable for identifying active labor onset because contractions increase inconsistently in intensity, frequency, and duration without an abrupt change distinguishing the phases 3, 1
- Montevideo Units and palpation of contractions provide limited diagnostic value for determining phase transitions 3, 1
Normal Progression Rates in Active Labor
Cervical Dilation Thresholds
- Nulliparous women: ≥1.2 cm/hour is normal; rates <1.2 cm/hour indicate protracted active phase 3, 1
- Multiparous women: ≥1.5 cm/hour is normal; rates <1.5 cm/hour indicate protracted active phase 3, 1
- These lower-limit thresholds have been validated by multiple investigators using objective instrumental methods including intrauterine pressure catheters 1
Monitoring Protocol
- Perform serial cervical examinations every 2 hours minimum to track dilation progress and identify the transition to active labor 1
- Plot cervical dilation on a partogram (graphical labor curve) to visualize the rate of progress and promptly detect deviations from normal 3, 1
- The dilatation pattern alone is reliable for prospectively identifying active phase onset and assessing normality of progression when graphed serially 3
Phases Within Active Labor
Acceleration and Maximum Slope Phase
- After entering active labor, dilation accelerates most markedly between 5 cm and 6 cm 4
- This represents the steepest portion of the labor curve where cervical change is most rapid 3
Deceleration Phase
- The deceleration phase occurs near complete dilation and is characterized by slowing cervical progress as the fetus descends 3
- Increased and painful contractions often signal the beginning of the deceleration phase and fetal descent 3
- Normal duration limits: ≤2–3 hours in nulliparous women and ≤1 hour in multiparous women 1
- A prolonged deceleration phase is strongly associated with cephalopelvic disproportion and second-stage abnormalities 1
Abnormal Active Labor Patterns
Protracted Active Phase
- Definition: Cervical dilation slower than the normal thresholds (nulliparous <1.2 cm/h; multiparous <1.5 cm/h) 3, 1
- This diagnosis cannot be made if the patient is still in latent phase, making accurate identification of active labor onset critical 3
- Cephalopelvic disproportion occurs in 25–30% of protracted active phase cases and must be evaluated before intervention 4
Arrest of Dilation
- Definition: Absence of cervical change despite adequate observation period and sufficient uterine contractions 1
- At 6–7 cm dilation, allowing arrest ≥4 hours is reasonable as it does not increase adverse neonatal outcomes 5
- At 8–9 cm dilation, arrest beyond 4 hours increases risks of cesarean delivery, chorioamnionitis, and adverse neonatal outcomes; a 2-hour window may be safer 4, 5
Clinical Pitfalls to Avoid
- Do not diagnose protracted active phase in women still in latent phase; misidentification leads to inappropriate interventions 3
- Do not rely on contraction assessment alone to determine labor phase; serial cervical examinations are essential 3, 1
- Do not assume a specific dilation measurement (e.g., 4 cm or 6 cm) automatically means active labor has begun; only an accelerating rate confirms the transition 1
- The latent phase may normally extend up to 20 hours in nulliparas and 14 hours in multiparas; premature diagnosis of labor abnormalities should be avoided 6