Before We Start
Preterm labor — regular contractions with cervical change before 37 weeks
The goal of treatment isn't always to stop labor entirely, it's usually to buy 48 hours: enough time for antenatal corticosteroids (betamethasone) to accelerate fetal lung maturity before delivery happens. Tocolytics (labor-suppressing medications) delay contractions long enough for the steroids to work.
Tocolytic Medications
Four drugs, four different mechanisms
Nifedipine (calcium channel blocker)
Relaxes uterine smooth muscle. Often a first-line choice given its relatively favorable side effect profile compared to other tocolytics.
Indomethacin (NSAID)
Inhibits prostaglandins that trigger contractions. Generally avoided after 32 weeks gestation, since it can cause premature closure of the fetal ductus arteriosus and reduce amniotic fluid volume (oligohydramnios) with prolonged use.
Terbutaline (beta-agonist)
Relaxes uterine muscle via beta-2 stimulation, similar to how albuterol relaxes airway smooth muscle. Can cause maternal tachycardia and tremor.
Magnesium sulfate (dual purpose)
Used both as a tocolytic and specifically for fetal neuroprotection, reducing the risk of cerebral palsy when given before an anticipated preterm birth. This dual role, and its real toxicity risk, makes it the most important tocolytic to know well.
💡 Memory Trick — Magnesium Sulfate Toxicity, In Order
Toxicity progresses predictably: loss of deep tendon reflexes comes first, before respiratory depression and before cardiac effects. Assess reflexes, respiratory rate, and urine output regularly during infusion. Report a respiratory rate under 12/min, urine output under 30 mL/hr, or absent reflexes immediately. Calcium gluconate is the antidote, keep it available at the bedside for the entire duration of the infusion, not just when trouble starts.
Diagnosis
Confirming true preterm labor
Regular contractions (roughly 6 or more per hour) plus documented cervical dilation and/or effacement change. Transvaginal ultrasound measures cervical length, a short cervix raises concern. Fetal fibronectin testing can help rule out imminent preterm birth when negative, a negative result has strong reassurance value.
🏥 Preterm Labor Scenarios — Apply What You've Learned
Three scenarios. Identify the correct nursing action.
1
Scenario: A patient at 34 weeks is receiving magnesium sulfate for preterm labor. The nurse assesses absent patellar reflexes and a respiratory rate of 10/min.
Correct action: Stop the infusion immediately and notify the provider. Prepare calcium gluconate, the antidote, and support respiratory status.
2
Scenario: A patient at 33 weeks in preterm labor is prescribed indomethacin for tocolysis, expected to continue for several days.
Concern: Indomethacin is generally avoided after 32 weeks and with prolonged use, due to risk of premature ductus arteriosus closure and oligohydramnios. This should be discussed with the provider.
3
Scenario: A patient at 29 weeks presents with regular contractions. The provider orders betamethasone and a tocolytic.
Correct interpretation: The goal is to buy roughly 48 hours for betamethasone to accelerate fetal lung maturity, not necessarily to stop labor entirely or indefinitely.
📌 NCLEX Application
NCLEX tests magnesium sulfate toxicity recognition and tocolytic-specific precautions.
Rules to know cold:
• Loss of deep tendon reflexes is the earliest sign of magnesium toxicity, before respiratory depression
• Calcium gluconate is the antidote for magnesium sulfate toxicity
• Indomethacin is generally avoided after 32 weeks (fetal ductus arteriosus risk)
• Antenatal corticosteroids (betamethasone) accelerate fetal lung maturity, buying time for tocolysis to work
• Magnesium sulfate is also used for fetal neuroprotection, reducing cerebral palsy risk
Common NCLEX trap: a question describes a nurse waiting for absent reflexes AND severe respiratory depression before acting on suspected magnesium toxicity — reflexes diminish first and should prompt action before respiratory status significantly worsens.
⚠️ The Trap — Treating All Tocolytics as Interchangeable
Students sometimes memorize "tocolytics stop preterm labor" as one undifferentiated category, missing that each drug carries its own specific monitoring requirements and contraindications.
Assuming indomethacin is safe at any gestational age, or forgetting that magnesium sulfate specifically requires reflex and respiratory monitoring (unlike nifedipine or terbutaline), can lead to a genuinely dangerous care gap.
NCLEX angle: "Which assessment is specifically required for a patient receiving magnesium sulfate that would NOT be required for a patient receiving nifedipine?" → Deep tendon reflex assessment — this is unique to magnesium sulfate's toxicity profile.
✓ Quick Self-Test
Answer before checking:
1. What is the goal of tocolytic therapy, in terms of time bought?
2. What is the earliest sign of magnesium sulfate toxicity?
3. What is the antidote for magnesium sulfate toxicity?
4. Why is indomethacin generally avoided after 32 weeks?
5. What is magnesium sulfate's second major use in preterm labor, beyond tocolysis?
Answers:
1. Roughly 48 hours, enough time for antenatal corticosteroids (betamethasone) to accelerate fetal lung maturity before delivery.
2. Loss of deep tendon reflexes.
3. Calcium gluconate.
4. It can cause premature closure of the fetal ductus arteriosus and reduce amniotic fluid volume (oligohydramnios) with prolonged use.
5. Fetal neuroprotection — reducing the risk of cerebral palsy when given before an anticipated preterm birth.
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