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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Preterm labor = regular contractions with cervical change before 37 weeks
- Rule out membrane rupture, infection, abruption, and fetal compromise
- Antenatal corticosteroids are used when preterm birth risk is significant
- Tocolysis is short-term, usually to gain 48 hours for steroids or transfer
- Magnesium sulfate before early preterm delivery provides fetal neuroprotection
Overview
Preterm labor is labor before 37 weeks with contractions causing cervical change. Management confirms true labor, identifies contraindications to prolonging pregnancy, and optimizes fetal outcomes with steroids, magnesium, transfer, or delivery.
Epidemiology
Preterm birth affects about 1 in 10 births in the United States. Risk factors include prior spontaneous preterm birth, short cervix, multifetal gestation, uterine anomalies, infection, smoking, substance use, and short interpregnancy interval.
Clinical Features
Symptoms
Regular contractions before 37 weeks
Pelvic pressure, low backache, cramps
Change in discharge or mucus plug
Fluid leakage suggests membrane rupture
Fever, uterine tenderness, foul discharge suggests infection
Bleeding or severe pain suggests abruption/previa
Signs
Cervical dilation or effacement
Short cervical length
Regular contractions on monitor
Maternal fever/fetal tachycardia/uterine tenderness
Nonreassuring fetal tracing
Investigations
First-line
Sterile speculum examinationAssess cervix, bleeding, discharge, membrane rupture; avoid digital exam if previa not excluded
Transvaginal cervical lengthLong cervix has high negative predictive value
Fetal monitoringAssess fetal heart rate and contractions
Second-line
Fetal fibronectinNegative test has high negative predictive value for delivery within 7-14 days
Infection evaluationUrinalysis/culture, STI testing, and assessment for chorioamnionitis
UltrasoundPresentation, growth, amniotic fluid, placenta, gestational age
Specialist
MFM consultationVery early gestation, recurrent preterm birth, multifetal pregnancy, short cervix
Neonatology consultationWhen preterm or periviable delivery possible
Management
ACOG Practice Bulletin: Management of Preterm Labor and ACOG Antenatal Corticosteroid Guidance1
Initial management
- Confirm gestational age, cervical change, membrane status, fetal status, and contraindications to tocolysis
- Treat identified infection
- Administer GBS prophylaxis if delivery possible and indicated
- Arrange transfer to appropriate neonatal facility when needed
2
Antenatal corticosteroids
- Betamethasone or dexamethasone reduces RDS, IVH, NEC, and mortality
- Classically for high delivery risk from 24 0/7 to 33 6/7 weeks
- May be considered in selected late preterm patients without prior steroids
- Do not delay urgent delivery for maternal/fetal instability solely to complete steroids
3
Tocolysis
- Use to delay delivery up to 48 hours for steroids, magnesium, or transfer
- Nifedipine is commonly used
- Indomethacin may be used before 32 weeks; avoid later due to ductus/oligohydramnios concerns
- Contraindicated with chorioamnionitis, significant bleeding, fetal compromise, severe preeclampsia, fetal demise, or advanced labor
4
Magnesium and prevention
- Magnesium sulfate for fetal neuroprotection when early preterm delivery imminent, commonly before 32 weeks
- Future pregnancy planning after spontaneous preterm birth or short cervix
- Vaginal progesterone or cerclage in selected patients
Complications
- Respiratory distress syndrome: Surfactant deficiency reduced by steroids
- Intraventricular hemorrhage: Risk increases with earlier gestation
- Necrotizing enterocolitis: Serious prematurity complication
- Chorioamnionitis: Contraindicates tocolysis
- Recurrent preterm birth: Prior spontaneous preterm birth increases recurrence
USMLE Step 2 CK Exam Tips
- 1Preterm labor requires contractions plus cervical change
- 2Negative fetal fibronectin = low risk of delivery soon
- 3Tocolysis buys 48 hours; it does not improve long-term neonatal outcomes alone
- 4Indomethacin avoided after 32 weeks
- 5Chorioamnionitis = antibiotics and delivery, not tocolysis
- 6Betamethasone is classic antenatal steroid
- 7Magnesium sulfate before early preterm delivery = fetal neuroprotection
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