Preterm Labor & PROM
High-yield STEP 2 CK lesson on preterm labor and PROM/PPROM: confirm the diagnosis at the bedside, then choose the next-best step by gestational age and infection status, anchored on the recurring answer set of corticosteroids, magnesium, GBS prophylaxis, and latency antibiotics. Draft was clinically strong; corrections tightened the indomethacin adverse-effect profile, specified betamethasone for the late-preterm course, clarified intrapartum GBS dosing, and added anaerobic coverage for chorioamnionitis at cesarean.
Framing: PTL vs PROM vs PPROM
Preterm birth (delivery before 37 0/7 weeks) is the leading cause of neonatal morbidity and mortality. Two overlapping scenarios dominate the boards. Preterm labor (PTL) is regular uterine contractions plus cervical change between 20 0/7 and 36 6/7 weeks. Prelabor rupture of membranes (PROM) is rupture of the amniotic sac before labor begins; when this occurs before 37 weeks it is preterm PROM (PPROM), which precedes roughly one-third of preterm births.
The tested task is almost always identical: confirm the diagnosis with the correct bedside test, then choose management by gestational age and by whether intra-amniotic infection (chorioamnionitis) is present. Four interventions form the recurring answer set — antenatal corticosteroids (lung maturity), magnesium sulfate (neuroprotection), GBS prophylaxis, and, for PPROM, latency antibiotics.
- PTL diagnosis: regular contractions + cervical change (dilation/effacement). TVUS cervical length <25 mm and a positive fetal fibronectin (fFN) raise risk; a negative fFN has strong negative predictive value (delivery unlikely in 7–14 days).
- PROM diagnosis (sterile speculum, NOT digital): pooling in the posterior fornix, Nitrazine paper turns blue (alkaline fluid, pH ~7.1–7.3), and ferning on dried microscopy. False-positive Nitrazine: blood, semen, bacterial vaginosis.
- Corticosteroids: 24 0/7–33 6/7 wk if delivery likely within 7 days (betamethasone or dexamethasone; may consider from 23 0/7 wk if resuscitation planned). A separate late-preterm course = betamethasone, 34 0/7–36 6/7 wk, only if intact membranes and no prior steroids.
- Magnesium sulfate: fetal neuroprotection (↓ cerebral palsy) when delivery expected <32 wk — it is NOT an effective tocolytic.
- Tocolytics buy ~48 h for steroids/transfer only: indomethacin <32 wk, nifedipine (any GA).
- GBS prophylaxis: intrapartum IV penicillin (ampicillin alternative) continued to delivery; give if GBS-positive or status unknown.
- PPROM latency antibiotics: IV ampicillin + erythromycin or azithromycin (~7 days). Avoid amoxicillin-clavulanate (neonatal necrotizing enterocolitis).
Vignette: A 27-year-old G2P1 at 30 weeks has regular painful contractions every 4 minutes for 2 hours. Cervix is 3 cm dilated, 80% effaced; membranes intact, fetal heart tracing reassuring, no fever.
Diagnosis: Preterm labor.
Next best steps (the bundle):
- Betamethasone IM — fetal lung maturity.
- Magnesium sulfate — neuroprotection (she is <32 wk).
- Tocolysis with indomethacin (preferred <32 wk) or nifedipine to delay ~48 h for steroid benefit.
- GBS prophylaxis (penicillin) — status is typically unknown in PTL, so treat empirically.
If instead 34–36 6/7 wk: give a late-preterm betamethasone course (if none prior and membranes intact), skip magnesium, and generally do not aggressively tocolyze — allow delivery if labor progresses.
Vignette A: A 32-year-old at 30 weeks reports a sudden gush of clear fluid. Speculum exam shows fluid pooling in the posterior fornix, Nitrazine turns blue, and ferning is seen. She is afebrile with a normal tracing. Dx: PPROM. Next step: admit for expectant management — betamethasone, magnesium (<32 wk), latency antibiotics (ampicillin + azithromycin), GBS prophylaxis, and infection/fetal surveillance toward 34 0/7 wk. Do NOT perform a digital exam (infection risk, shortens latency).
Vignette B: Two days later she develops T 38.6°C, uterine tenderness, fetal tachycardia (175), and foul discharge. Dx: Intra-amniotic infection (chorioamnionitis). Next step — regardless of gestational age: broad-spectrum antibiotics (ampicillin + gentamicin; add clindamycin/metronidazole if cesarean) plus prompt delivery (induce/augment; cesarean only for obstetric indications). Chorioamnionitis contraindicates tocolysis and expectant management.
Tocolytic comparison
| Agent | Class | Preferred window | Key cautions |
|---|---|---|---|
| Indomethacin | NSAID (COX inhibitor) | <32 wk | Premature ductus arteriosus constriction (risk ↑ with gestational age — avoid >32 wk); oligohydramnios with prolonged use |
| Nifedipine | Calcium-channel blocker | Any GA (often first-line) | Maternal hypotension; caution combining with magnesium |
| Terbutaline | β2-agonist | Short-term only | Maternal tachycardia, pulmonary edema; black-box: no prolonged use |
| Magnesium sulfate | — | <32 wk | Neuroprotection, NOT an effective tocolytic; toxicity → ↓DTRs → respiratory depression (antidote: calcium gluconate) |
The classic speculum triad — Pool • Nitrazine • Fern:
- Pool — visible amniotic fluid pooling in the posterior fornix (most reliable clinical sign).
- Nitrazine — pH paper turns blue; alkaline amniotic fluid (~7.1–7.3) vs the normally acidic vagina. Think “blue = the bag broke.”
- Fern — dried fluid crystallizes into a fern / arborization pattern under the microscope.
Blue-but-not-ruptured (Nitrazine false positives): blood, semen, bacterial vaginosis — all alkaline. Always use a sterile speculum, not a digital exam, in suspected PPROM.
Prevention & next-step traps
Prevention: a short cervix (<25 mm on TVUS) or a prior spontaneous preterm birth → vaginal progesterone; a prior spontaneous preterm birth plus short cervix (<24 wk) may warrant cervical cerclage. (17-OHP caproate / Makena was withdrawn from the US market in 2023 for lack of efficacy.)
High-yield next-step traps:
- Term PROM (≥37 wk) → induce labor (oxytocin) rather than expectantly manage — reduces chorioamnionitis (TERMPROM).
- Chorioamnionitis, abruption, or nonreassuring fetal status → deliver; withhold tocolysis and any expectant delay.
- Give corticosteroids even if delivery may occur before the second dose — a partial course still helps.
- Magnesium = neuroprotection, not a tocolytic.
- PPROM ≥34 0/7 wk → delivery is generally recommended.
Practice OB/GYN now
Board-style questions, spaced-repetition flashcards, and a Socratic AI tutor — free to start.