Preterm Labor

1) Antenatal corticosteroids

2) Tocolytics (short-term delay ~48 hours)

3) GBS prophylaxis (intrapartum) — at-a-glance

Note: Always follow current guidance, and the manufacturer’s labeling. CDC Prevent Group B Strep App for Obstetric Providers available https://www.cdc.gov/group-b-strep/hcp/clinical-guidance/prevent-group-b-strep-app-for-obstetric-providers.html

When to give antenatal corticosteroids

• Goal timing: Give when preterm birth is likely within 7 days. Benefit is greatest when birth occurs 24 hours to 7 days after treatment, but steroids may still be beneficial even when the first dose is given within 24 hours of birth.
• ACOG (core window): Recommend a single course for patients 24+0 to 33+6 weeks who are at risk of preterm delivery within 7 days (including those with ruptured membranes and multiple gestations).
• RCOG (core window): Offer a course to patients 24+0 to 34+6 weeks when imminent preterm birth is anticipated (e.g., established preterm labor, PPROM, or planned preterm birth).
• ACOG (late preterm, select patients): Betamethasone may be considered for patients 34+0 to 36+6 weeks at risk of preterm birth within 7 days, particularly when they have not previously received a course of antenatal corticosteroids.
• Periviable gestations (individualized, if neonatal resuscitation planned): ACOG notes steroids may be considered starting at 23+0 to 23+6 weeks based on shared decision-making. A joint ACOG/SMFM Practice Advisory discusses updated evidence at 22+0 to 22+6 weeks in the setting of planned resuscitation after counseling.
• Planned cesarean near term (RCOG): For planned cesarean birth 37+0 to 38+6, an informed discussion should occur about potential benefits vs risks (e.g., uncertain effect on some respiratory outcomes and possible neonatal harms such as hypoglycemia).
• Repeat (“rescue”) course (ACOG): A single repeat course may be considered for patients <34+0 weeks who remain at risk of preterm delivery within 7 days when the prior course was given >14 days earlier (and may be considered as early as 7 days depending on the clinical scenario).

Practical note: Birth should not be delayed solely to complete steroids if the indication for delivery is impacting maternal or fetal health.

See the At-a-glance dosing table for quick regimens.

Progesterone Indicated for

• Singleton, no prior preterm birth, with cervical length ≤ 25 mm.
• Singleton with prior spontaneous preterm birth with cervical length < 25 mm versus cerclage.
• Consider exam-indicated cerclage for singleton or twin before or at 24 weeks.

Options

• 200 mg progesterone vaginal suppository or capsule  nightly until 36 weeks OR
• Vaginal progesterone 8% gel (90 mg daily)

• Prometrium® (micronized progesterone): PDF
• Other progesterone gel products: use manufacturer labeling per product.

Intrapartum prophylaxis is indicated if:

• Previous infant with invasive GBS disease.
• GBS bacteriuria during any trimester of the current pregnancy.
• Positive GBS screening culture during current pregnancy (unless planned cesarean without labor and without ROM).
• Unknown GBS status and any of the following:
  • Delivery at <37 weeks
  • ROM >18 hours
  • Intrapartum temperature >100.4°F (>38.0°C)
• Intrapartum NAAT or vaginal/rectal culture positive for GBS.

Abbreviation: NAAT = nucleic acid amplification tests. If intrapartum NAAT is negative but other intrapartum risk factors are present, prophylaxis is indicated.

Penicillin-allergic patients at high risk for anaphylaxis: history of immediate hypersensitivity (e.g., angioedema, urticaria) including penicillin-related anaphylaxis; also asthma or other diseases making anaphylaxis more dangerous, or beta-blocker therapy.

For the dosing summary, see the GBS prophylaxis at-a-glance table.

“Overall, the evidence supports the use of first-line tocolytic treatment with beta-adrenergic receptor agonists, calcium channel blockers, or NSAIDs for short-term prolongation of pregnancy (up to 48 hours) to allow for the administration of antenatal steroids” [9G]

Tocolysis is generally not indicated for

• Gestation > 34 weeks
• Fetal death in utero
• Fetal malformation where palliative care only is planned
• Suspected fetal compromise warranting delivery
• Placental abruption (unless minor and remote from term)
• Chorioamnionitis
• Pre-eclampsia
• Negative fetal fibronectin (unless cervical change occurs)

Indomethacin (Indocin®)

Nonsteroidal anti-inflammatory (prostaglandin synthetase inhibitor / COX inhibitor).
Package insert (label): Manufacturer/FDA labeling (searchable)

• Acute treatment of preterm labor at <32 weeks

Dose: Initial dose 50–100 mg PO. May repeat in 1 hour if no decrease in contraction frequency, then 25–50 mg every 6 hours.

Contraindications / cautions: peptic ulcer, hematologic dysfunction, kidney/liver disease, pregnancy at or after 32 weeks, IUGR, chorioamnionitis, oligohydramnios, ductal-dependent cardiac defects, twin transfusion syndrome; use with caution in asthma.

Maternal side effects: GI irritation, heartburn, nausea/vomiting; possible hypertension with beta blockers.
Fetal side effects: premature ductal closure, decreased renal function/oligohydramnios; NEC in preterm newborns and PDA in newborn.

(Supplied as 25 & 50 mg capsules; 25 mg/5 mL suspension; 50 mg rectal suppositories.)

Nifedipine (Adalat®, Procardia®)

Calcium channel blocker.
Package insert : Procardia® PI (PDF)

• Acute treatment of preterm labor

Dosing examples:

• 20 mg PO; may repeat in 30 minutes if contractions persist; after controlled, 20 mg every 3 to 8 hours
• OR 10 mg PO every 20 minutes × 3 doses, then 20 mg PO every 4 to 6 hours

Maximum dose 160 mg/day. Discontinue after 48 hours, pulse >120 bpm, or BP <90/60 mm Hg.

Contraindicated: allergy to nifedipine, sick sinus syndrome, secondary/tertiary heart block, hypotension, hepatic dysfunction.

Side effects: tachycardia, flushing, headache, dizziness, nausea, vasodilatation, hypotension in hypovolemia; maternal hepatotoxicity reported.
(Supplied as: 10, 20 mg immediate-release capsules; 30, 60, 90 mg extended-release capsules.)

Terbutaline (Brethine®)

Beta₂-adrenergic receptor agonist sympathomimetic.
Package insert (label): Manufacturer/FDA labeling (searchable)

• Acute treatment of preterm labor

Dose: 0.25 mg SC every 15–30 minutes up to total 0.5 mg in 4 hours, then 0.25 mg every 3 hours. Discontinue after 48 hours or pulse >120 bpm.

Contraindicated: cardiac disease, poorly controlled diabetes, antepartum hemorrhage, hypertension, hyperthyroidism, fetal distress, severe preeclampsia, severe IUGR, chorioamnionitis, abruption.

Side effects: chest pain, dyspnea, tachycardia, palpitations, tremor, headaches, hypokalemia, hyperglycemia, nausea/vomiting, nasal stuffiness, pulmonary edema, fetal tachycardia. (Legacy supply: 1 mg/mL ampules.)

Magnesium sulfate

Fetal exposure to magnesium sulfate in women at risk of preterm delivery appears to reduce the risk of cerebral palsy. The appropriate total dosage, infusion period, need for retreatment, and therapeutic window for neuroprotection are not known (legacy text) [14,15].

Regimen A

Loading dose of 4 g by infusion pump over 30 minutes, followed by continuous intravenous infusion at 1 g per hour until birth. Discontinue if delivery is no longer imminent or after a maximum of 24 hours of therapy (legacy text) [16].

Regimen B

Loading dose of 6 g by infusion pump over 20 to 30 minutes, followed by continuous intravenous infusion at 2 g per hour. Discontinue after 12 hours if delivery is no longer considered imminent. If threat of delivery recurs after 6 or more hours, then repeat bolus (legacy text) [6].

Supplied as

Magnesium sulfate 50% (5 g/10 mL = 4 mEq/mL) must be diluted to ≤20% before IV infusion.

UPDATED 12/4/2025