6.2 Preterm Labor, Premature Rupture of Membranes & Cervical Insufficiency

Preterm Labor (PTL)

Preterm birth is the leading cause of neonatal mortality and long-term neurologic morbidity. Preterm labor is strictly defined as the presence of regular uterine contractions (≥4 in 20 minutes or ≥8 in 60 minutes) accompanied by progressive cervical change (dilation or effacement) before 37 weeks of gestation.

Risk Factors

Major risk factors include a prior history of spontaneous preterm birth (the strongest predictor), short cervical length (<25 mm), multiple gestations, smoking, substance abuse, maternal stress, low socioeconomic status, and intrauterine infection (chorioamnionitis).

Diagnosis and Evaluation

Accurate diagnosis is challenging but crucial to avoid unnecessary interventions. Evaluation involves:

• Sterile speculum exam: To assess for premature rupture of membranes (PROM) and visually inspect the cervix.
• Transvaginal ultrasound (TVUS): To measure cervical length. A length >30 mm has a high negative predictive value for preterm birth.
• Fetal Fibronectin (fFN): A glycoprotein acting as "glue" at the maternal-fetal interface. Its presence in cervicovaginal secretions between 22 and 34 weeks indicates a disruption. A negative fFN test is highly reassuring that delivery will not occur within the next 7-14 days.

Management of Preterm Labor

The primary goal of managing PTL (<34 weeks) is to delay delivery long enough (typically 48 hours) to administer therapies that improve neonatal outcomes.

1. Antenatal Corticosteroids: Administration of betamethasone (12 mg IM every 24 hours for 2 doses) or dexamethasone (6 mg IM every 12 hours for 4 doses) between 24 and 33 6/7 weeks is the most impactful intervention. Corticosteroids significantly reduce the incidence of respiratory distress syndrome (RDS), intraventricular hemorrhage (IVH), necrotizing enterocolitis (NEC), and neonatal mortality.

2. Tocolytic Therapy: Tocolytics are uterine relaxants used to delay delivery for up to 48 hours. They do not significantly reduce the overall rate of preterm birth but allow time for corticosteroid benefit and maternal transport.

• Indomethacin: A cyclooxygenase (COX) inhibitor. First-line for <32 weeks. Avoid after 32 weeks due to the risk of premature closure of the fetal ductus arteriosus and oligohydramnios.
• Nifedipine: A calcium channel blocker. First-line for 32-34 weeks. Side effects include maternal hypotension and flushing.
• Terbutaline: A beta-adrenergic agonist. Given subcutaneously, often for short-term acute tocolysis. Side effects include maternal tachycardia, hypokalemia, and pulmonary edema.

3. Magnesium Sulfate for Neuroprotection: Administered for impending preterm delivery before 32 weeks to reduce the incidence and severity of cerebral palsy.

4. Prophylaxis against GBS: Intravenous Penicillin G or Ampicillin is given to prevent neonatal Group B Streptococcus infection if the status is positive or unknown.

Premature Rupture of Membranes (PROM)

PROM is the rupture of the amniotic sac before the onset of labor. If this occurs before 37 weeks, it is termed Preterm PROM (PPROM).

Diagnosis

Diagnosis is clinical, confirmed by a sterile speculum exam:

• Pooling: Clear fluid in the posterior vaginal fornix.
• Nitrazine Test: Amniotic fluid is alkaline (pH 7.1-7.3), turning the yellow Nitrazine paper blue.
• Fern Test: Amniotic fluid creates a characteristic ferning pattern on a microscope slide due to NaCl crystallization.
• Ultrasound: May show oligohydramnios, supporting the diagnosis. Note: Digital cervical exams should be strictly avoided to minimize the risk of ascending infection.

Management of PPROM

Management depends on gestational age and clinical status.

• <34 Weeks (Expectant Management): If there are no signs of infection or fetal compromise, the goal is to prolong pregnancy. Admit to the hospital. Administer a 7-day course of latency antibiotics (Ampicillin and Erythromycin IV for 48 hours, followed by oral Amoxicillin and Erythromycin for 5 days). This prolongs the latency period and reduces neonatal infections. Administer corticosteroids and consider magnesium sulfate if <32 weeks.
• ≥34 Weeks: The risk of infection (chorioamnionitis) outweighs the risks of prematurity. Delivery (induction of labor) is recommended.
• Chorioamnionitis: Regardless of gestational age, clinical chorioamnionitis (maternal fever, uterine fundal tenderness, maternal/fetal tachycardia, purulent fluid) mandates broad-spectrum IV antibiotics (e.g., Ampicillin + Gentamicin) and immediate delivery.

Cervical Insufficiency

Cervical insufficiency (incompetence) is the inability of the cervix to retain a pregnancy in the second trimester, absent clinical contractions or labor.

• Presentation: Often presents as painless cervical dilation, pelvic pressure, or bulging membranes in the second trimester (typically 16-24 weeks).
• Risk Factors: Prior cervical conization/LEEP, mechanical dilation (D&C), structural uterine anomalies, collagen disorders (Ehlers-Danlos).
• Management: Cervical cerclage (a purse-string suture placed around the cervix) is the primary treatment. It can be placed prophylactically (history-indicated at 12-14 weeks) or as a rescue procedure (ultrasound-indicated for short cervix or physical exam-indicated for early dilation).