Labor, Preterm

Key Features

• Preterm birth defined as birth between 20 0/7 and 36 6/7 weeks' gestation

• Spontaneous preterm labor responsible for at least two-thirds of preterm births

• Preterm, regular, rhythmic contractions 5 minutes apart

• Cervical dilatation, effacement, or both occur

• Most common reason for antepartum hospitalization

Clinical Findings

• Current cusp of viability is 23–25 weeks' gestation; infants born prior to 23 weeks rarely survive

• Rates of infant death and long-term neurologic impairment are inversely related to gestational age at birth

• About two-thirds of preterm births are "late preterm births," which occur between 34 0/7 weeks and 36 6/7 weeks; good outcomes are expected for "late preterm births"

• Major risk factors for spontaneous preterm labor include

  • A past history of preterm birth

  • A short cervical length as measured by transvaginal ultrasound

• Other known risk factors include

  • African ancestry

  • Multiple gestation

  • Intrauterine infection

  • Substance abuse

  • Smoking

  • Periodontal disease

  • Socioeconomic deprivation

  • Ruptured membranes, which precede numerous preterm births

Diagnosis

• Fetal fibronectin measurement in cervicovaginal specimens can differentiate true from false labor

• A level < 50 ng/mL has a negative predictive value of 93–97% for delivery in 7–14 days among women with a history of preterm delivery currently having contractions

• However, fetal fibronectin is not recommended as a screening test in asymptomatic women because of its low sensitivity

Treatment

• Limited activity and bed rest

  • Frequently recommended despite the fact that evidence has failed to demonstrate improved outcomes in these women

  • Additionally, and paradoxically, such recommendations may place a woman at an increased risk to deliver preterm

• Women in preterm labor should receive antimicrobial prophylaxis against group B streptococcus

  • Penicillin G, 5 million units intravenously as a loading dose and then 2.5–3 million units intravenously every 4 hours until delivery

    • In penicillin-allergic patients not at high risk for anaphylaxis, cefazolin, 2 g intravenously as an initial dose and then 1 g intravenously every 8 hours until delivery

    • In patients at high risk for anaphylaxis, vancomycin, 20 mg/kg intravenously every 8 hours until delivery

  • Clindamycin, 900 mg intravenously every 8 hours until delivery, can also be used after a group B streptococcal isolate has been confirmed to be susceptible to clindamycin

• Corticosteroids

  • A single short-course of corticosteroids should be administered to promote fetal lung maturity when preterm birth is anticipated between 23 and 34 weeks gestation

    • Betamethasone, 12 mg intramuscularly repeated once 24 hours later or

    • Dexamethasone, 6 mg intramuscularly repeated every 12 hours for four doses

  • A single repeat course should be considered in women who are at risk for preterm delivery within the next 7 days, and whose prior dose of antenatal corticosteroids was administered more than 14 days previously

  • Rescue course corticosteroids could be provided as early as 7 days from the prior dose, if indicated by the clinical scenario

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