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Summary Table

Category	Details
Risk Factors	Nulliparity, advanced maternal age, obesity (BMI > 30), macrosomia, short stature, gestational diabetes, post-term pregnancy, excessive weight gain, prior history of labor dystocia
Etiology	Dysfunction in one or more of the 3 P's: Power (inadequate uterine contractions), Passenger (fetal size, malpresentation, malposition), Passage (contracted pelvis, soft tissue obstruction)
Patient Presentation	Woman in active labor whose cervical dilation or fetal descent has slowed or completely stopped despite ongoing contractions
Classic Physical Exam	Cervical dilation that plateaus on the partograph, fetal station that fails to descend, possible caput succedaneum or molding on vaginal exam, Bandl ring (retraction ring) in obstructed labor
Key Diagnostic Results	Serial cervical exams plotted on a partograph crossing the alert/action line; intrauterine pressure catheter (IUPC) showing Montevideo units < 200 in adequate trial; ultrasound for fetal weight estimation and position
Management	Protraction: amniotomy + oxytocin augmentation. Arrest: adequate trial of oxytocin (Montevideo units > 200 for at least 4 hours with no change, or 6 hours if contractions remain inadequate) then cesarean delivery if no progress. Second stage arrest: operative vaginal delivery (vacuum/forceps) if criteria met, otherwise cesarean
Keywords	"Failure to progress," "prolonged latent phase," "protraction disorder," "arrest disorder," "crossing the action line," "inadequate contractions," "Montevideo units," "active phase arrest"

1. Pathophysiology

Normal labor depends on the coordinated interaction of three fundamental variables known as the 3 P's: Power, Passenger, and Passage. Abnormal labor occurs when any one of these components is deficient, and the exam will frequently test your ability to identify which P is the culprit based on the clinical scenario.

Power refers to the strength and coordination of uterine contractions. Normal active labor requires contractions that generate sufficient intrauterine pressure to dilate the cervix. This is quantified using Montevideo units (MVU), calculated by summing the peak amplitude above baseline (in mmHg) of each contraction over a 10-minute window. A value of 200 MVU or greater is considered adequate. When contractions are hypotonic (too weak) or incoordinate (unorganized), the cervix dilates slowly or not at all. This is the most common and most correctable cause of labor abnormalities, which is why oxytocin augmentation is the first intervention tested on exams.

Passenger refers to the fetus, including its size, lie, presentation, and position. Macrosomia (estimated fetal weight > 4,000 g, or > 4,500 g in diabetic mothers) increases the risk of labor arrest because the fetal head or shoulders cannot navigate the pelvic inlet and midpelvis. Occiput posterior (OP) position is a high-yield exam concept: the fetal head presents with a larger diameter, leading to protracted descent even with adequate contractions. Malpresentations such as brow or face presentation similarly increase the presenting diameter and cause dystocia.

Passage refers to the bony pelvis and soft tissues. A contracted pelvis (e.g., android or platypelloid pelvic types) narrows the available space for fetal descent. Soft tissue causes include cervical scarring, large fibroids in the lower uterine segment, or a distended bladder. If the passage is the limiting factor, augmentation with oxytocin will not resolve the problem, and continued augmentation against an obstructed pelvis risks uterine rupture. This is the key reason the exam tests whether you can differentiate "power" problems (treatable with oxytocin) from "passage" or "passenger" problems (requiring cesarean delivery).

The partograph is the clinical tool that connects pathophysiology to diagnosis. By plotting cervical dilation and fetal descent against time, you can objectively identify when labor has deviated from normal. Modern labor management has shifted from the classic Friedman curve to evidence from the Zhang/ACOG Safe Prevention of the Primary Cesarean guidelines, which recognize that labor before 6 cm can be physiologically slower than previously thought.

2. Classification and Clinical Manifestation

Phases of Labor

Phase	Definition	Expected Progress
First Stage: Latent Phase	Onset of regular contractions to 6 cm dilation	Highly variable; up to 20 hours (nullipara) or 14 hours (multipara) considered within normal range
First Stage: Active Phase	6 cm to complete dilation (10 cm)	Nullipara: at least 1 cm/hr once labor is established; Multipara: generally faster (1.2 to 1.5 cm/hr)
Second Stage	Complete dilation to delivery of the infant	Nullipara: up to 3 hours without epidural, 4 hours with epidural; Multipara: up to 2 hours without, 3 hours with
Third Stage	Delivery of infant to delivery of placenta	Expected within 30 minutes

Abnormal Labor Patterns

Disorder	Phase Affected	Definition	Most Likely Cause
Prolonged Latent Phase	First stage, latent	> 20 hours (nullipara) or > 14 hours (multipara)	Sedation, early epidural, unripe cervix, false labor
Protraction of Dilation	First stage, active	Dilation < 1 cm/hr in active phase (after 6 cm)	Inadequate uterine contractions (power), OP position
Arrest of Dilation	First stage, active	No cervical change for 4 hours with adequate contractions (> 200 MVU), or 6 hours with inadequate contractions despite oxytocin	Cephalopelvic disproportion (CPD), malpresentation, true power failure
Protraction of Descent	Second stage	Descent < 1 cm/hr (nullipara) or < 2 cm/hr (multipara)	OP position, large fetus, inadequate pushing effort
Arrest of Descent	Second stage	No descent for > 1 hour of active pushing (nullipara without epidural), > 2 hours (with epidural)	CPD, persistent OP, deep transverse arrest

Protraction vs. Arrest

Feature	Protraction	Arrest
Defining concept	Labor is progressing, but too slowly	Labor has completely stopped
Implication	Often a power problem; trial of augmentation is appropriate	May be a passenger or passage problem; must rule out CPD before continuing
Management bias	Augment and observe	Time-limited trial of adequate contractions, then cesarean if no change

3. Diagnostic Workup

Test	Role	Key Finding
Serial cervical examination	Best initial assessment	Plotted on partograph; identifies protraction or arrest pattern
Partograph	Screening and monitoring tool	Cervical dilation crossing the alert or action line indicates abnormal progress
External tocodynamometry	Initial contraction assessment	Confirms presence and frequency of contractions, but cannot measure strength
Intrauterine pressure catheter (IUPC)	Confirmatory for adequacy of contractions	Montevideo units; > 200 MVU = adequate contractions
Ultrasound	Adjunct	Estimated fetal weight, fetal lie, presentation, and position (especially to confirm OP)
Clinical pelvimetry	Adjunct	Diagonal conjugate (< 11.5 cm suggests inlet contraction), prominence of ischial spines (midpelvis narrowing)

The best initial step in evaluating suspected abnormal labor is a cervical examination to establish current dilation, effacement, station, and fetal position. This must be compared to prior exams to determine whether there is protraction (slow progress) or arrest (no progress). The partograph is the standard visual tool for this purpose.

If there is concern that contractions are inadequate, the next step is to quantify contraction strength. External tocometry can tell you frequency and duration but not amplitude. If the clinical picture suggests a power problem (i.e., contractions feel weak, cervix is slowly dilating, and there is no evidence of CPD), placement of an IUPC is the confirmatory test. Montevideo units below 200 indicate inadequate contractions, which gives you a clear rationale to augment with oxytocin before labeling the labor as arrested.

Ultrasound is used to evaluate the passenger. If the cervix has arrested despite adequate contractions, you need to assess fetal weight (to rule out macrosomia), confirm the presentation is vertex, and check for OP position. An estimated fetal weight above 5,000 g in a non-diabetic mother or above 4,500 g in a diabetic mother is an indication for cesarean delivery regardless of labor progress.

An important exam concept: you cannot diagnose active phase arrest until the patient is at least 6 cm dilated. Before 6 cm, slow progress is considered prolonged latent phase, and the management is different (therapeutic rest or augmentation, not cesarean).

4. Management and Treatment

Clinical Scenario	First-Line Management	Next Step if No Response	Endpoint for Cesarean
Prolonged latent phase	Therapeutic rest (morphine 10 to 15 mg IM or IV sedation) or oxytocin augmentation	If no cervical change after rest/augmentation, re-evaluate for false labor	Cesarean is rarely indicated for latent phase alone
Protraction of dilation	Amniotomy + oxytocin augmentation; target > 200 MVU	Place IUPC to confirm adequate contractions; continue augmentation	If adequate contractions (> 200 MVU) for 4 hours with no cervical change, proceed to cesarean
Arrest of dilation	Ensure adequate contractions via IUPC; oxytocin if MVU < 200	Cesarean delivery	4 hours of adequate contractions with no change, OR 6 hours of augmentation if contractions remain inadequate
Protraction of descent	Encourage maternal pushing, position changes, consider oxytocin if contractions suboptimal	Assess for OP position (manual rotation may be attempted)	Extended duration without progress leads to operative delivery
Arrest of descent	Evaluate for operative vaginal delivery (vacuum or forceps) if criteria met	Cesarean delivery	No descent after 1 to 2 hours of active pushing (varies by parity and epidural status)

Prolonged Latent Phase

The latent phase is the most variable part of labor, and the exam frequently tests that cesarean delivery should not be performed solely for a prolonged latent phase. Management options include therapeutic rest (morphine sulfate 10 to 15 mg IM with promethazine 25 mg to allow the patient to sleep; most will awaken in active labor) or augmentation with oxytocin (starting at 1 to 2 mU/min, increased every 15 to 30 minutes). Amniotomy can also be performed if membranes are intact and the head is well-applied to the cervix.

Active Phase Protraction and Arrest

Once the patient is at least 6 cm, active phase criteria apply. The approach is sequential and exam-testable:

Amniotomy if membranes are intact. This alone can increase contraction strength and accelerate dilation.
Oxytocin augmentation, starting at 1 to 2 mU/min IV, titrated upward every 15 to 30 minutes. The goal is to achieve at least 3 contractions per 10 minutes with MVU of 200 or greater.
IUPC placement to objectively measure contraction adequacy when external monitoring is equivocal.
If adequate contractions (MVU > 200) are documented for at least 4 hours with no cervical change, active phase arrest is diagnosed, and cesarean delivery is indicated.
If contractions remain inadequate despite oxytocin (MVU remains < 200), you must allow up to 6 hours of augmentation before declaring arrest and proceeding to cesarean.

These time thresholds (4 hours adequate, 6 hours inadequate) come from ACOG guidelines and are heavily tested.

Second Stage Disorders

For protracted or arrested descent, first ensure the patient is actively pushing with adequate technique. Position changes (e.g., squatting, lateral positioning) can help. If the fetal head is at station +2 or below and all criteria for operative vaginal delivery are met (known position, adequate pelvis, vertex presentation, fully dilated cervix, engaged head, ruptured membranes, empty bladder), vacuum extraction or forceps can be attempted. If operative vaginal delivery fails or criteria are not met, cesarean delivery is performed.

Contraindications and Cautions

Oxytocin is contraindicated in the setting of a prior classical (vertical) uterine incision, active placenta previa, vasa previa, cord prolapse, and non-reassuring fetal heart tracing (category III). Augmentation with oxytocin in a patient with a prior low transverse cesarean is permitted for trial of labor after cesarean (TOLAC) but requires continuous fetal monitoring due to the risk of uterine rupture.

Tachysystole (more than 5 contractions in 10 minutes) during oxytocin administration requires immediate reduction or discontinuation of oxytocin, repositioning the patient to the left lateral decubitus position, and administering IV fluid bolus. If fetal heart rate abnormalities accompany tachysystole, terbutaline 0.25 mg subcutaneous can be given as a tocolytic to relax the uterus.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
False labor (Braxton-Hicks contractions)	Irregular contractions without cervical change; may mimic prolonged latent phase	No cervical dilation or effacement change on serial exams over 2 to 4 hours; contractions typically resolve with ambulation or hydration
Chorioamnionitis	Can present with stalled labor and maternal fever, potentially confused with arrest	Maternal fever > 38.0 C, fetal tachycardia, uterine tenderness, purulent amniotic fluid; labor may still be augmented but with antibiotics (ampicillin + gentamicin), and this does not change the arrest diagnosis
Uterine rupture	Sudden cessation of contractions, loss of fetal station	Acute onset of severe abdominal pain, loss of prior fetal station (fetal parts palpable abdominally), non-reassuring fetal tracing (prolonged deceleration), vaginal bleeding; requires emergent cesarean
Obstructed labor (CPD)	Overlaps with arrest of dilation/descent	History of adequate contractions (MVU > 200) with absolutely no progress; caput and significant molding on exam; Bandl ring on physical exam or imaging; fetal head not engaged despite full dilation
Cervical stenosis or scarring	Can mimic failure of cervical dilation	History of cervical procedures (cone biopsy, LEEP); the cervix feels rigid and fibrotic rather than simply undilated; may respond to gentle dilation
Precipitous labor	Opposite end of the spectrum but can distract in exam stems about "abnormal labor"	Total labor duration < 3 hours from onset to delivery; associated with uterine tachysystole, not protraction; risk of postpartum hemorrhage and fetal injury

6. Traps and High-Yield Pearls

The single most tested trap in labor abnormalities is performing cesarean delivery too early. The exam will present a woman at 4 or 5 cm who has been laboring for many hours with slow progress and ask you what the next step is. The correct answer is almost never cesarean at this point, because active phase criteria do not apply until 6 cm. Before 6 cm, the management is therapeutic rest or augmentation, not surgical delivery. Students who choose cesarean in this scenario are penalized for not understanding the revised definition of active labor.

The second major trap involves the distinction between adequate and inadequate contractions when diagnosing arrest. If a vignette describes a patient at 7 cm with no cervical change for 3 hours, but does not mention MVU or contraction adequacy, the next best step is not cesarean. The correct answer is to place an IUPC or ensure contractions are adequate before declaring arrest. You must demonstrate adequate power before blaming the passenger or passage.

A third high-yield pearl: the exam loves to test OP position as the sneaky cause of protracted descent. The vignette will describe a patient who is fully dilated, pushing for a long time, with the fetal head "at 0 station" and the fontanelle palpable anteriorly on vaginal exam (meaning the posterior fontanelle is posterior, confirming OP). The correct recognition of OP position may lead to a question about manual rotation or operative vaginal delivery.

Finally, remember that amniotomy is always performed before oxytocin augmentation in most management algorithms. If the vignette gives you a patient with intact membranes and poor progress, amniotomy is the correct first step before escalating to IV oxytocin. Getting the sequence right (amniotomy then oxytocin, not the reverse) is a frequently tested ordering question.

The core competency being tested is your ability to differentiate correctable causes of abnormal labor from those requiring surgical delivery, apply the correct time thresholds before diagnosing arrest, and avoid premature cesarean section while also recognizing when continued augmentation becomes unsafe.

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