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Preterm Labor

By Julie S. Moldenhauer, MD

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Patient Education

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), corticosteroids (if gestational age is < 34 wk), and possibly magnesium sulfate (if gestational age is < 32 wk). Antistreptococcal antibiotics are given pending negative anovaginal culture results.

Preterm labor may be triggered by

A cause may not be evident.

Prior preterm delivery and cervical incompetence increase the risk.

Premature labor can increase risk of intraventricular hemorrhage in neonates; intraventricular hemorrhage may result in neurodevelomental disability (eg, cerebral palsy).


  • Clinical evaluation

Diagnosis of preterm labor is based on signs of labor and length of the pregnancy.

Anovaginal cultures for group B streptococci are done, and prophylaxis is appropriately initiated. Urinalysis and urine culture are done to check for cystitis and pyelonephritis. Cervical cultures are done to check for STDs if suggested by clinical findings.

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

  • Magnesium sulfate if gestational age is < 32 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

  • Magnesium sulfate

  • A calcium channel blocker

  • Prostaglandin inhibitors

No tocolytic is clearly the first-line choice; choice should be individualized to minimize adverse effects.

Magnesium sulfate is commonly used and is typically well-tolerated.

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.

IV magnesium sulfate should be considered in pregnancies < 32 wk. In utero exposure to the drug appears to reduce the risk of severe neurologic dysfunction (eg, due to intraventricular hemorrhage), including cerebral palsy, in neonates.


If the fetus is < 34 wk, women are given corticosteroids unless delivery is imminent. One of the following may be used:

  • Betamethasone 12 mg IM q 24 h for 2 doses

  • Dexamethasone 6 mg IM q 12 h for 4 doses

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 magnesium sulfate, a Ca channel blocker, or, if the fetus is ≤ 32 wk, a prostaglandin inhibitor.

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

  • Consider magnesium sulfate if the fetus is < 32 wk.

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

* This is the Professional Version. *