Definition of Labor
Labor is defined as regular uterine contractions accompanied by progressive cervical change (dilation and effacement), with active labor beginning when the cervix reaches 6 cm dilation in contemporary practice. 1
Components of Labor Definition
Essential Criteria for Labor Diagnosis
- Labor requires both regular uterine contractions AND demonstrable cervical change (dilation and/or effacement), as contractions alone do not define true labor. 2
- Serial vaginal examinations at least every 2 hours are necessary to document progressive cervical change and distinguish true labor from false labor. 3
- Uterine contraction assessment (palpation or Montevideo Units) has limited value for diagnosing labor phases, as contractions inconsistently increase in intensity, frequency, and duration throughout the first stage. 3
Stages of Labor
First Stage of Labor:
- Extends from onset of labor through complete cervical dilation (10 cm). 3
- Divided into latent phase and active phase. 3
- The latent phase shows negligible or absent cervical dilation slope. 3
- The active phase begins at various degrees of dilation when the rate of cervical change transitions from the flat slope of latent phase to more rapid progression, identified by the inflection point on the cervical dilation curve. 3
- Active phase continues until full cervical dilation, with most cervical dilation occurring during this period. 3
- May take up to 36 hours in nulliparous women and is shorter in multiparous women. 3
Second Stage of Labor:
- Extends from complete cervical dilation (10 cm) to delivery of the fetus. 3, 4
- Involves descent and rotation of the fetal presenting part through the birth canal. 4
- May take 30 minutes to 2 hours. 3
- Descent typically begins during the deceleration phase of cervical dilation as the cervix is drawn upward around the fetal presenting part. 4
Third Stage of Labor:
- Defined as the time period between delivery of the fetus through delivery of the placenta. 5
- May take up to 1 hour if not actively managed. 3
Special Considerations in Third Trimester with Compromised Fetal Position
Fetal Malposition Impact
- Fetal malpositions (occiput posterior, occiput transverse) and malpresentations (brow) are underlying factors for abnormal labor patterns including protracted dilation, arrest of dilation, and failure of descent. 3
- Malpositions may prevent normal labor progression despite adequate uterine contractions. 4
Clinical Assessment Requirements
- Accurate identification of fetal position, attitude, and degree of cranial molding is essential when assessing labor progress with compromised fetal position. 4
- Evaluation must include assessment of pelvic architecture and capacity to accommodate the fetus. 4
- Cephalopelvic disproportion must be ruled out before attempting vaginal delivery when labor abnormalities are identified with fetal malposition. 3
Contraindications to Labor
- Unfavorable fetal positions or presentations that are undeliverable without conversion prior to delivery (such as transverse lies) are absolute contraindications to oxytocin use and vaginal delivery. 6
- Cord presentation or prolapse, total placenta previa, and vasa previa contraindicate vaginal delivery. 6
Common Pitfalls
- Do not rely on contraction patterns alone to diagnose labor phases, as contractile patterns have yielded little to facilitate differentiating real labor from false labor. 3
- Do not assume active labor begins at a specific cervical dilation measurement (such as 4 cm), as the active phase begins at various degrees of dilation when the rate of change accelerates. 3
- In women with compromised fetal position, do not proceed with oxytocin augmentation or operative vaginal delivery without thorough cephalopelvimetric assessment, as the frequency of cephalopelvic disproportion is considerably greater with labor abnormalities. 3
- Prolonged deceleration phase (from 8-10 cm) with fetal malposition strongly predicts shoulder dystocia if vaginal delivery occurs, requiring heightened vigilance. 3, 7