What is labor?

What is Labor?

Labor is defined as regular uterine contractions that produce progressive cervical change (dilation and effacement), distinguishing it from false labor where contractions occur without cervical progression. 1, 2

Core Definition and Components

Labor consists of coordinated uterine contractions that drive two essential processes:

• Cervical dilation and effacement - The cervix undergoes progressive opening from closed to 10 cm (complete dilation) 1, 3
• Fetal descent through the birth canal - Propulsion of the fetus occurs through the pelvis, supplemented by maternal bearing-down efforts in the second stage 1

The key distinguishing feature is that contractions must produce documented cervical change to qualify as true labor, as opposed to false labor where contractions occur without cervical progression. 3, 4

Stages of Labor

First Stage: Latent and Active Phases

Latent Phase:

• Extends from labor initiation until the active phase begins 3
• Characterized by a relatively flat slope of cervical dilation 1
• Normal duration up to approximately 20 hours in nulliparas and 14 hours in multiparas 3
• The cervix undergoes rapid remodeling, softening, thinning, and modest dilation in preparation for active labor 3

Active Phase:

• Begins when the rate of cervical dilation accelerates from the flat latent phase slope to a more rapid progression, regardless of the specific centimeter of dilation achieved 1, 2
• Identified through serial vaginal examinations at least every 2 hours to detect when dilation rate increases 1, 2
• Normal dilation rates are ≥1.2 cm/hour for nulliparas and ≥1.5 cm/hour for multiparas 1, 2
• Continues until the cervix reaches full dilation (10 cm) 1

Critical Pitfall: While some guidelines suggest active labor begins at 5-6 cm dilation, this fixed threshold contradicts well-documented observations that many women enter active phase before reaching this dilation, while others remain in latent phase beyond it. 5 The transition is defined by accelerating dilation rate, not a specific centimeter measurement. 1

Second Stage

• Begins at complete cervical dilation (10 cm) and ends with delivery of the fetus 2
• Recommended duration allows 2-3 hours for completion 2

Third Stage

• Extends from fetal delivery to placental delivery 2
• Normal duration up to 1 hour if not actively managed 2

Physiologic Mechanisms

Uterine Contractility:

• Uterine contractions create the primary driving force for cervical dilation and fetal propulsion 1
• Contractions increase inconsistently in intensity, frequency, and duration throughout the first stage, with no abrupt change marking active phase onset 1
• Assessing contractions by palpation or Montevideo Units (≥200 units considered normal) has limited value for determining labor phase, as contractile patterns vary widely in both normal and dysfunctional labor 1

Cervical Changes:

• The cervix undergoes extensive collagen and ground substance remodeling, particularly during the latent phase 3
• The dilatation pattern is the only reliable indicator for prospectively identifying active phase onset and normal progression when graphed serially 1, 5

Clinical Assessment

Essential Monitoring:

• Serial vaginal examinations (minimum every 2 hours) to track cervical dilation rate 1, 2
• Graphing dilation on a partogram to visualize labor progression 1, 2
• Continuous fetal heart rate monitoring 2
• Assessment of uterine contraction patterns 2

Key Clinical Principle: The diagnosis of true labor versus false labor, and the identification of labor phase transitions, relies primarily on documenting progressive cervical change through serial examinations rather than contraction assessment alone. 1, 3, 4