4.4 Preterm Labor & Preterm Prelabor Rupture of Membranes

Preterm Labor: Assessment & Clinical Management Bundle

Preterm labor is defined as regular uterine contractions accompanied by cervical dilation and/or effacement occurring between 20 0/7 and 36 6/7 weeks gestation.

Clinical Evaluation & Diagnostic Biomarkers

• Uterine Contractions: Regular, painful contractions (≥4 every 20 minutes or ≥8 in 60 minutes).
• Cervical Examination: Documented progressive cervical change, or initial cervical dilation ≥2 cm and/or effacement ≥80%.
• Transvaginal Ultrasound Cervical Length (TVUS CL): Highly sensitive screening tool. A cervical length >30 mm has a high negative predictive value (>95%) for delivery within 14 days, whereas a cervical length <20 mm strongly predicts impending preterm delivery.
• Fetal Fibronectin (fFN): An extracellular matrix glycoprotein found at the decidual-chorionic interface. Disruption of the interface releases fFN into cervicovaginal secretions. Tested between 22 0/7 and 34 6/7 weeks:
  • Negative fFN (<50 ng/mL): High negative predictive value (>95% to 99% probability that delivery will not occur within the next 7 to 14 days), allowing safe de-escalation of interventions and avoidance of unnecessary hospitalizations.
  • Testing Precautions: Cervicovaginal specimens must be collected prior to digital examination, transvaginal ultrasound, or sexual intercourse within the preceding 24 hours, which yield false-positive results. Gross bleeding also invalidates the test.

The Evidence-Based Preterm Labor Care Bundle (24 0/7 to 33 6/7 Weeks)

When active preterm labor is diagnosed in pregnancies prior to 34 weeks, an immediate multimodal bundle of evidence-based pharmacotherapies is initiated:

                  [ PRETERM LABOR CLINICAL BUNDLE: 24 0/7 – 33 6/7 WEEKS ]
                                              │
     ┌────────────────────┬───────────────────┴───────────────────┬────────────────────┐
     ▼                    ▼                                       ▼                    ▼
[ Antenatal Steroids ] [ Short-Term Tocolysis ]            [ Neuroprotection ]   [ GBS Prophylaxis ]
Betamethasone 12mg IM  Oral Nifedipine or                  Magnesium Sulfate     IV Penicillin G
q24h x 2 doses         Indomethacin (for ≤48h)             4g load, 1g/h         5 million units load,
Reduces RDS, IVH,      Allows steroid completion           Reduces Cerebral      then 2.5–3.0 MU q4h
NEC, & mortality       and maternal transport              Palsy (<32 weeks)     until delivery

1. Antenatal Corticosteroids

• Indications: All pregnant individuals between 24 0/7 and 33 6/7 weeks gestation at risk of preterm delivery within 7 days. Also considered in the periviable period (22 0/7–23 6/7 weeks) and in late preterm gestations (34 0/7–36 6/7 weeks) if delivery is expected within 7 days and steroids were not administered previously.
• Regimens:
  • Betamethasone: 12 mg IM administered every 24 hours for two doses (preferred), OR
  • Dexamethasone: 6 mg IM administered every 12 hours for four doses.
• Mechanism & Benefits: Corticosteroids induce fetal pulmonary type II pneumocytes to synthesize and release surfactant. Administration dramatically reduces the incidence and severity of neonatal respiratory distress syndrome (RDS), intraventricular hemorrhage (IVH), necrotizing enterocolitis (NEC), and neonatal mortality.

2. Short-Term Tocolytic Pharmacotherapy

Tocolytics are utilized strictly for short-term suppression of uterine contractions (up to 48 hours) to permit the administration of antenatal corticosteroids and facilitate maternal transfer to a facility equipped with an appropriate-level neonatal intensive care unit. Tocolytics do not prolong pregnancy to term.

• Nifedipine (First-Line Calcium Channel Blocker): Oral loading dose of 20 to 30 mg, followed by 10 to 20 mg orally every 4 to 6 hours for up to 48 hours. Well-tolerated; monitor for maternal hypotension, peripheral flushing, and headache.
• Indomethacin (First-Line Prostaglandin Synthetase Inhibitor): Oral or rectal loading dose of 50 to 100 mg, followed by 25 to 50 mg orally every 6 hours for a maximum of 48 hours.
  • Strict Gestational Age Limitation: Reserved for pregnancies <32 0/7 weeks. Use beyond 32 weeks or for longer than 48 hours carries severe fetal risks: premature constriction of the ductus arteriosus (causing fetal pulmonary hypertension) and oligohydramnios (due to reduced fetal renal blood flow).

3. Magnesium Sulfate for Fetal Neuroprotection

• Indication: Administered to individuals between 24 0/7 and 31 6/7 weeks gestation who are in active preterm labor with anticipated delivery within 12 to 24 hours.
• Dosing: Intravenous loading dose of 4 grams over 20 to 30 minutes, followed by a continuous infusion of 1 gram/hour until delivery or for a maximum of 24 hours.
• Benefit: Neuroprotection stabilizes neuronal membranes and microvascular cerebral blood flow, producing a substantial reduction in the incidence and severity of cerebral palsy and gross motor dysfunction in surviving preterm infants.

4. Group B Streptococcus (GBS) Chemoprophylaxis

Because GBS culture results are rarely available in preterm labor, empiric intravenous antibiotic prophylaxis is initiated upon admission:

• First-Line: Intravenous Penicillin G (5 million units IV loading dose, followed by 2.5 to 3.0 million units IV every 4 hours until delivery).
• Penicillin-Allergic: Cefazolin (mild allergy) or Clindamycin/Vancomycin (severe anaphylactic allergy, based on susceptibility testing).

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Preterm Premature Rupture of Membranes (PPROM)

Preterm premature rupture of membranes is defined as rupture of fetal chorioamniotic membranes prior to 37 0/7 weeks gestation and before the onset of regular uterine contractions. It precedes one-third of all preterm births.

Confirmation of Membrane Rupture

Evaluation must be conducted via sterile speculum examination. Digital cervical examination is strictly contraindicated because digital exams introduce vaginal flora through the open cervical os into the amniotic cavity, markedly shortening the latency period and accelerating the development of intraamniotic infection.

• Classic Diagnostic Triad:
  1. Direct Pooling: Direct visualization of clear amniotic fluid pooling in the posterior vaginal fornix or egress of fluid from the external cervical os upon coughing or fundal pressure.
  2. Nitrazine Paper Test: Amniotic fluid is alkaline (pH 7.0 to 7.3), turning yellow nitrazine paper deep blue (vaginal secretions are normally acidic, pH 3.8 to 4.5). False-positive results occur in the presence of semen, blood, cervical mucus, or bacterial vaginosis.
  3. Microscopic Ferning Test: Air-dried fluid examined under low-power microscopy displays a distinctive arborization or ferning pattern driven by the crystallization of sodium chloride in the presence of amniotic proteins and estrogen.

Clinical Management Protocol (24 0/7 to 33 6/7 Weeks)

In the absence of infection or fetal distress, expectant inpatient management is maintained until 34 0/7 weeks to maximize fetal maturation:

• Hospital Admission: Strict inpatient bed-to-chair rest and serial monitoring for intraamniotic infection (daily maternal temperature, maternal pulse, fetal heart rate, and uterine tenderness).
• Antenatal Corticosteroids: Single course of betamethasone (12 mg IM q24h x 2 doses).
• Latency Antibiotic Regimen (7-Day Course): Prolongs latency, reduces neonatal sepsis, and prevents chorioamnionitis:
  • Initial 48 Hours (IV): Intravenous Ampicillin (2 grams IV every 6 hours) PLUS intravenous Erythromycin (250 mg IV every 6 hours) or oral Azithromycin (1 gram PO single dose).
  • Subsequent 5 Days (Oral): Oral Amoxicillin (250 mg PO every 8 hours) PLUS oral Erythromycin base (333 mg PO every 8 hours).
  • Absolute Contraindication: Amoxicillin-clavulanic acid (Augmentin) is strictly contraindicated for latency therapy because it significantly increases the risk of neonatal necrotizing enterocolitis (NEC).
• Delivery Timing: Planned induction of labor is initiated at 34 0/7 weeks gestation. Immediate delivery is indicated regardless of gestational age if overt intraamniotic infection (chorioamnionitis), placental abruption, umbilical cord prolapse, or non-reassuring fetal status develops.