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Preterm Labor

by Julie S. Moldenhauer, MD

Labor (contractions resulting in cervical change) that begins before 37 wk gestation is considered preterm. Risk factors include premature rupture of membranes, uterine abnormalities, infection, cervical incompetence, prior preterm birth, multifetal pregnancy, and placental abnormalities. Diagnosis is clinical. Causes are identified and treated if possible. Management typically includes bed rest, tocolytics (if labor persists), and corticosteroids (if gestational age is < 34 wk). Antistreptococcal antibiotics are given pending negative anovaginal culture results.

Preterm labor may be triggered by premature rupture of membranes, chorioamnionitis (see Intra–Amniotic Infection), or another ascending uterine infection; group B streptococci are a common cause of such infections. Preterm labor may also be due to multifetal pregnancy, fetal or placental abnormalities, uterine abnormalities, pyelonephritis, or some sexually transmitted diseases (STDs); a cause may not be evident. Prior preterm delivery and cervical incompetence increase the risk.


  • Clinical evaluation

Cervical cultures are done to check for causes suggested by clinical findings (eg, pyelonephritis, STDs). Anovaginal cultures for group B streptococci are done, and prophylaxis is appropriately initiated.

Most women with a presumptive diagnosis of preterm labor do not progress to delivery.


  • Antibiotics for group B streptococci, pending anovaginal culture results

  • Tocolytics

  • Corticosteroids if gestational age is < 34 wk

Bed rest and hydration are commonly used initially.

Antibiotics effective against group B streptococci are given pending negative anovaginal cultures. Choices include the following:

  • For women without penicillin allergy: Penicillin G 5 million units IV followed by 2.5 million units q 4 h or ampicillin 2 g IV followed by 1 g q 4 h

  • For women with penicillin allergy but a low risk of anaphylaxis (eg, maculopapular rash with prior use): Cefazolin 2 g IV followed by 1 g q 8 h

  • For women with penicillin allergy and an increased risk of anaphylaxis (eg, bronchospasm, angioneurotic edema, or hypotension with prior use, particularly within 30 min of exposure): Clindamycin 900 mg IV q 8 h or erythromycin 500 mg IV q 6 h if anovaginal cultures show susceptibility; if cultures document resistance or results are unavailable, vancomycin 1 g IV q 12 h

If the cervix dilates, tocolytics (drugs that stop uterine contractions) can usually delay labor for at least 48 h so that corticosteroids can be given to reduce risks to the fetus. Tocolytics include Mg sulfate, Ca channel blockers, and prostaglandin inhibitors. No tocolytic is clearly the first-line choice; choice should be individualized to minimize adverse effects. Mg sulfate is commonly used and is typically well-tolerated (see Mg sulfate). Prostaglandin inhibitors may cause transient oligohydramnios. They are contraindicated after 32 wk gestation because they may cause premature narrowing or closure of the ductus arteriosus.

If the fetus is < 34 wk, women are given corticosteroids: betamethasone 12 mg IM q 24 h for 2 doses or dexamethasone 6 mg IM q 12 h for 4 doses unless delivery is imminent. These corticosteroids accelerate maturation of fetal lungs and decrease risk of neonatal respiratory distress syndrome, intracranial bleeding, and mortality.

A progestin may be recommended in future pregnancies for women who have a preterm delivery to reduce the risk of recurrence. This treatment is initiated during the 2nd trimester and continued until just before delivery.

Key Points

  • Do anovaginal cultures for group B streptococci and cultures to check for any clinically suspected infections that could have triggered preterm labor (eg, pyelonephritis, STDs).

  • Treat with antibiotics effective against group B streptococci pending culture results.

  • If the cervix dilates, consider tocolysis with Mg sulfate, a Ca channel blocker, or, if the fetus is ≤ 32 wk, a prostaglandin inhibitor.

  • Give a corticosteroid if the fetus is < 34 wk.

  • In future pregnancies, consider giving a progestin to prevent recurrence.

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