Abnormal Labor & Fetal Heart Rate Monitoring
A board-focused walkthrough of abnormal labor — the 3 Ps, protraction versus arrest criteria (including the ≥200 MVU rule and when to choose cesarean over oxytocin) — paired with NICHD fetal heart rate interpretation, the three decelerations, and their next-best-step management.
Overview: The 3 Ps and Why We Watch the Strip
Labor progresses through three stages: stage 1 (onset to full 10-cm dilation), stage 2 (full dilation to delivery of the fetus), and stage 3 (delivery of the placenta). Stage 1 divides into a latent phase (up to ~6 cm) and an active phase (6–10 cm).
Abnormal labor (dystocia) comes from a problem in one of the 3 Ps — Power (uterine contractions), Passenger (fetal size/position), or Passage (maternal pelvis). Boards want you to separate protraction (labor too slow) from arrest (labor stopped), decide when to augment with oxytocin versus proceed to cesarean, and correctly read the fetal heart rate (FHR) tracing.
Continuous FHR monitoring (cardiotocography) is a screening test for fetal hypoxemia: a reassuring tracing reliably predicts a well-oxygenated fetus (high negative predictive value), whereas an abnormal tracing is nonspecific and triggers a stepwise intrauterine resuscitation response rather than immediate delivery.
- Active phase begins ~6 cm; historic normal dilation ≥1.2 cm/hr (nulliparous), ≥1.5 cm/hr (multiparous)
- Prolonged latent phase: >20 h nulliparous, >14 h multiparous → therapeutic rest ± oxytocin; not an indication for cesarean by itself
- Active-phase arrest = ≥6 cm + ruptured membranes + no cervical change for ≥4 h with adequate contractions OR ≥6 h with inadequate contractions
- Adequate contractions = ≥200 Montevideo units (MVU) — sum of contraction peak amplitudes above baseline over 10 min, measured by an intrauterine pressure catheter (IUPC)
- Second-stage arrest (no descent): nulliparous >3 h (>4 h with epidural); multiparous >2 h (>3 h with epidural)
- Next step logic: inadequate power → oxytocin ± amniotomy; adequate power (≥200 MVU) with no progress → cesarean delivery
- Baseline 110–160 bpm. Tachycardia >160 → maternal fever/chorioamnionitis, hypoxia, beta-agonists; bradycardia <110
- Moderate variability (6–25 bpm) is the single best predictor of normal fetal acid–base status; absent/minimal variability is worrisome
- Accelerations are reassuring: ≥15 bpm for ≥15 sec (≥32 wk); ≥10 bpm for ≥10 sec (<32 wk)
- Category I (normal): baseline 110–160 + moderate variability + no late or variable decels (early decels and accelerations may be present)
- Category III (abnormal): sinusoidal pattern OR absent variability WITH recurrent late decels, recurrent variable decels, or bradycardia
- Category II = everything in between (indeterminate) → continued evaluation and intrauterine resuscitation
- Sinusoidal pattern (smooth sine wave, 3–5 cycles/min) → severe fetal anemia (fetomaternal hemorrhage, Rh alloimmunization)
The Three Decelerations
| Deceleration | Timing & shape | Cause | Significance / next step |
|---|---|---|---|
| Early | Gradual; nadir coincides with contraction peak (mirror image) | Fetal head compression (vagal) | Benign — no intervention |
| Variable | Abrupt drop (onset→nadir <30 s), "V/W" shape, variable timing | Umbilical cord compression | Reposition; amnioinfusion if recurrent |
| Late | Gradual; nadir after contraction peak, delayed recovery | Uteroplacental insufficiency | Worrisome — intrauterine resuscitation |
Vignette: A 29-year-old G1P0 at 40 weeks with ruptured membranes and an epidural has been 6 cm dilated for 4 hours. An intrauterine pressure catheter shows 220 MVU; the cervix is unchanged. FHR is category I.
Diagnosis: Active-phase arrest with adequate contractions (≥200 MVU for ≥4 h with no change).
Next best step: Cesarean delivery. Because uterine power is already adequate, adding oxytocin will not help — arrest with adequate contractions is a classic cesarean indication.
Contrast: If the IUPC showed <200 MVU, the answer would flip to oxytocin augmentation ± amniotomy first, since the problem is inadequate power, not a true arrest.
Vignette: A 33-year-old G3P2 at 39 weeks receiving oxytocin develops recurrent late decelerations with minimal variability. Maternal BP is normal.
Diagnosis: Late decels = uteroplacental insufficiency → fetal hypoxemia. With minimal (not absent) variability plus recurrent lates, this is Category II — it would be Category III only if variability were absent.
Next best step — intrauterine resuscitation (do these together): stop oxytocin, place mother in left lateral decubitus, give an IV fluid bolus, and apply supplemental O2; correct maternal hypotension if present. If the tracing does not improve → expedite delivery (cesarean, or operative vaginal delivery if fully dilated at low station).
Buzzword: for tachysystole (>5 contractions/10 min) with fetal compromise, give a tocolytic (terbutaline).
VEAL CHOP — the classic FHR pattern-to-cause pairing:
- Variable decel → Cord compression
- Early decel → Head compression
- Accelerations → Okay (adequate Oxygenation)
- Late decel → Placental (uteroplacental) insufficiency
Management link: variable/cord → reposition ± amnioinfusion; late/placental → intrauterine resuscitation (stop oxytocin, left lateral, IV fluids, O2), deliver if persistent; early/head → benign, no action.
Protraction vs Arrest — Disorder Cheat Sheet
| Disorder | Definition | Management |
|---|---|---|
| Prolonged latent phase | >20 h (nulliparous) / >14 h (multiparous) | Therapeutic rest ± oxytocin; not cesarean alone |
| Protracted active phase | Dilation slower than normal but still progressing | Oxytocin ± amniotomy |
| Active-phase arrest | ≥6 cm, membranes ruptured, no change ≥4 h (adequate MVU) or ≥6 h (inadequate) | Cesarean if ≥200 MVU; otherwise oxytocin first |
| Second-stage arrest | No descent: nullip >3 h (>4 h epidural); multip >2 h (>3 h epidural) | Operative vaginal delivery or cesarean |
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