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Preterm Labor

BASICS

  • Definition: Regular contractions with cervical change or dilation β‰₯2 cm, occurring between 20 0/7 and 36 6/7 weeks’ gestation.
  • Preterm birth: Birth between 20 0/7 and 36 6/7 weeks.

EPIDEMIOLOGY

  • Leading cause of perinatal morbidity/mortality in the U.S.
  • Incidence: 10–15% of pregnancies; 10% of all U.S. births are preterm (2019).
  • Causes:
  • Spontaneous preterm labor: 40–50%
  • Preterm premature rupture of membranes (PPROM): 25–35%
  • Medically indicated: 25–35%
  • Disparities: Non-Hispanic Black patients have 50% higher risk.

ETIOLOGY AND PATHOPHYSIOLOGY

  • Premature myometrial activation: Formation of gap junctions.
  • Abnormal placental events: Implantation, abruption, ischemia.
  • Systemic/local inflammation/infection: UTI, intraamniotic, GBS, bacterial vaginosis (present in 25–40%).
  • Uterine factors: Overdistension (multiple gestation, polyhydramnios), structural anomalies.
  • Trauma, fetal, or maternal complications.
  • Genetics: Familial tendency, gene-environment interactions.

RISK FACTORS

  • Prior preterm birth (strongest risk factor)
  • Short interpregnancy interval (<18 months)
  • Low BMI or low weight
  • Substance abuse (tobacco, cocaine)
  • Cervical length <25 mm
  • Cervical insufficiency, prior surgery
  • Uterine anomalies, large fibroids
  • Maternal infections, multiple gestation, IUGR, polyhydramnios
  • Placenta previa, abruption, bleeding
  • Socioeconomic disadvantage, Black race, chronic stress

GENERAL PREVENTION

  • Education: Counsel all pregnant patients, emphasize in at-risk groups.
  • Cervical screening:
  • Routine anatomic scan (18–22 6/7 weeks); TVUS if short cervix suspected.
  • Women with prior preterm birth: TVUS at 16–24 weeks.
  • Progesterone:
  • Vaginal or IM in singleton pregnancy with history of preterm birth or short cervix (<25 mm).
  • Note: 17-hydroxyprogesterone approval withdrawn (PROLONG study).
  • Insufficient data for use in multiple gestation.
  • Cerclage:
  • Consider for cervical insufficiency, short cervix with history.
  • Lifestyle:
  • Optimize interpregnancy interval, smoking cessation.

DIAGNOSIS

Clinical

  • Regular contractions (20–36 6/7 weeks) with cervical dilation/effacement, or dilation β‰₯2 cm.

History

  • Regular contractions/cramping, low back pain, lower abdominal pain, pelvic pressure
  • Vaginal discharge, bleeding, amniotic fluid leak

Physical Exam

  • Sterile speculum: rupture, cultures, cervical check
  • Bimanual: only if membranes intact

Differential Diagnosis

  • Braxton-Hicks, round ligament pain, UTI/vaginitis, appendicitis, nephrolithiasis, lumbosacral pain

DIAGNOSTIC TESTS & INTERPRETATION

  • fFN (fetal fibronectin):
  • Negative result (>97% NPV for delivery in 14 days)
  • Positive result: modestly increased risk (PPV 13–30%)
  • Transvaginal US: Cervical length/funneling
  • Cultures: Gonorrhea, chlamydia, wet prep, GBS (as indicated)
  • Urinalysis, urine culture
  • CBC, Kleihauer-Betke if abruption suspected
  • US: Fetal position, gestational age, fluid, fetal weight
  • pH/fern test: ROM
  • Monitor contractions: External tocodynamometry

TREATMENT

General Measures

  • Treat risk factors, hospitalize if IV tocolysis needed

Medications

  • Corticosteroids (24–34 weeks; consider at 23 weeks if at risk for delivery):
  • Betamethasone 12 mg IM Γ—2 (24h apart)
  • Dexamethasone 6 mg IM q12h Γ—4
  • Rescue course: Consider if <34 weeks, last steroids >14 days ago, delivery expected soon
  • Tocolysis (to allow corticosteroids, 48h window):
  • Nifedipine (CCB): 20 mg PO load, then 10–20 mg q4–6h Γ—48h
    • Avoid with MgSOβ‚„, monitor BP
    • Contraindications: hypotension, aortic insufficiency
  • Indomethacin (NSAID): 50–100 mg PO load, then 25–50 mg q6–8h (use up to 32 wks)
    • Contraindications: platelet/bleeding disorders, GI ulcers, asthma
  • Terbutaline (Ξ²-agonist): SC, avoid oral/long-term use
    • Contraindications: cardiac disease, uncontrolled DM
  • Contraindications to tocolysis: Severe preeclampsia/eclampsia, hemorrhage, advanced labor, IUGR, chorioamnionitis, fetal distress, lethal anomaly, PPROM
  • Magnesium sulfate:
  • For neuroprotection if delivery <32 weeks
  • Antibiotics:
  • GBS prophylaxis if indicated

Second Line

  • Magnesium sulfate: Not for tocolysis; for neuroprotection.
  • Antibiotics: Not for prolonging pregnancy if membranes intact.

SURGERY/PROCEDURES

  • Cerclage: For cervical insufficiency/shortened cervix with history, rescue if needed.

ONGOING CARE

  • Admission: For IV therapy, continuous monitoring, cervical assessments.
  • Follow-up: Weekly visits with contraction/cervical monitoring or US if high risk.
  • Routine maintenance tocolysis not recommended.

PATIENT EDUCATION

  • Seek care for regular contractions >1 hour, bleeding, fluid leakage, decreased fetal movement.

PROGNOSIS

  • If membranes ruptured: 3–7 days to delivery common.
  • If intact: 20–50% deliver preterm.

COMPLICATIONS

  • Labor resistant to tocolysis, pulmonary edema, intraamniotic infection

ICD-10 CODES

  • O60.03 Preterm labor without delivery, third trimester
  • O60.0 Preterm labor without delivery
  • O60.02 Preterm labor without delivery, second trimester

CLINICAL PEARLS

  • Tocolysis is for short-term delay to allow corticosteroid administration and transfer to higher level care.
  • Magnesium sulfate is recommended for neuroprotection in imminent delivery <32 weeks.