Anesthesiology APPLIED Exam › Obstetric anesthesia

Preterm labor: tocolysis, magnesium and the preterm cesarean

Obstetric anesthesia·~5 min read· Physician-reviewed✓ Sourced · 4

Stem

A 26-year-old G1 at 28 weeks, 60 kg, presented in preterm labor 14 hours ago. She received nifedipine 30 mg over the first hour, then 10 mg every 6 hours, and the first dose of betamethasone 12 mg IM. A magnesium sulfate infusion was started 2 hours ago, 4 g load then 1 g/h. Labor has advanced, the fetus is breech, and the obstetrician plans cesarean, likely through a vertical uterine incision. Blood pressure is 94/56, heart rate 112.

Question

What do her medications mean for your anesthetic, and how will you anesthetize her?

Ideal answer

HeadlineA spinal is still my choice, but she is vasodilated, magnesium-loaded and carrying a 28-week baby: I plan for hypotension, weakness and a fragile neonate.

Keep the magnesium running through delivery, because before 32 weeks it reduces cerebral palsy.¹
Phenylephrine infusion before the block, because nifedipine and magnesium together drop her pressure further.¹
Spinal, or CSE for a long vertical incision, because it spares her a general anesthetic on top of magnesium.
If general: full intubating dose, cautious redosing, quantitative monitoring, because magnesium potentiates neuromuscular blockade.²
Neonatal team in the room, because magnesium near delivery can depress the newborn and her steroid course is incomplete.²

Watch for: reflexes, respiratory rate and urine output for magnesium toxicity; atony after delivery from both tocolytics.

Breaking down the answer

What tocolysis is for — 48 hours to finish steroids and give magnesium; it does not itself improve neonatal outcome, and maintenance therapy is ineffective.¹ Changes if: delivery is inevitable; tocolysis stops and the steroid course stays incomplete.
Nifedipine — 20–30 mg load, then 10–20 mg every 4–6 hours; hypotension and reduced contractility.¹ Contraindicated below 90/50 mmHg or with cardiac disease.³ Changes if: preload-dependent lesions such as aortic insufficiency; it is avoided.¹
Indomethacin, the other first-line option — 50–100 mg load, then 25 mg every 4–6 hours for 48 hours; not after 32 weeks, because of ductal constriction, oligohydramnios and necrotizing enterocolitis.¹^,3 Changes if: platelet dysfunction or renal disease.¹^,3
Steroids — betamethasone 12 mg IM twice, 24 hours apart, between 24 0/7 and 33 6/7 weeks.³ Changes if: a prior course more than 7 days ago with delivery likely within 7 days; a rescue course is suggested.¹
Magnesium levels — reflexes fade above 4 mEq/L; at about 10 mEq/L reflexes are lost, respiratory paralysis and heart block appear.² Changes if: oliguria, because it is renally cleared.
Magnesium and sedatives — opioids, hypnotics and anesthetics add CNS depression.² Changes if: I add IV opioid; I reduce it and monitor breathing closely.

Follow-up question

The baby is delivered under spinal and handed to the neonatal team. Thirty minutes later in recovery she is drowsy, her respiratory rate is 8, patellar reflexes are absent, and she has made 10 mL of urine in the last hour. What do you do?

Follow-up answer

HeadlineThis is magnesium toxicity from falling clearance; I say so and stop the infusion.

I support her breathing with oxygen and bag-mask ventilation, intubating if she does not recover. I give calcium gluconate 1 g (10 mL of 10%) IV over 3–10 minutes, repeated if needed,⁴^,2 and furosemide to speed excretion;⁴ dialysis if her kidneys have failed. I send a magnesium level and creatinine, and check the block level, because a high spinal looks similar. I reassess reflexes, breathing and urine every 15 minutes; magnesium restarts at a lower rate only once the level falls.⁴

Pitfalls that cost points

Stopping magnesium at incision to protect the anesthetic.
Reducing the rocuronium intubating dose because of magnesium.
Giving indomethacin at 33 weeks.

Rapid-fire

Q: Which magnesium regimens give neuroprotection? A: 6 g then 2 g/h for up to 12 hours (the BEAM trial regimen, common in the US), or 4 g over 20 minutes then 1 g/h for up to 24 hours.¹

Q: Is magnesium an effective tocolytic? A: No; the evidence does not show it works as one, so it is given only for neuroprotection here.³

Q: What magnesium level can be fatal? A: Above 12 mEq/L.²

Q: What happens with magnesium beyond 5–7 days? A: Fetal hypocalcemia and bone abnormalities.²

Q: What is a normal serum magnesium? A: 1.5–2.5 mEq/L; 2.5–5 mEq/L usually controls seizures.²

Numbers to know

Item	Value
Magnesium neuroprotection	Before 32 weeks; 6 g then 2 g/h, or 4 g then 1 g/h¹
Nifedipine	20–30 mg load, 10–20 mg every 4–6 h¹
Indomethacin	50–100 mg load, 25 mg every 4–6 h; not after 32 weeks¹
Betamethasone	12 mg IM × 2, 24 h apart³
Reflexes lost / respiratory paralysis	~10 mEq/L²
Fatal magnesium	>12 mEq/L²

Sources

1.Risk Stratification and Management of Preterm Birth (summary of ACOG Practice Bulletin 171 and Committee Opinions) · The ObG Project · obgproject.comSecondary summary of ACOG guidance: tocolytic agents and doses, magnesium neuroprotection regimens, antenatal corticosteroid indications.
2.Magnesium sulfate injection, prescribing information · Pfizer (FDA label) · labeling.pfizer.com
3.Management of Patients at Risk for Preterm Delivery · Delaware Division of Public Health (citing ACOG/SMFM Committee Opinion 455) · dhss.delaware.govBetamethasone 12 mg IM x2, 24 h apart; nifedipine and indomethacin regimens and contraindications.
4.Practice Bulletin No. 222: Gestational Hypertension and Preeclampsia (excerpt) · ACOG (hosted by Kansas Perinatal Quality Collaborative), 2020 · kansaspqc.kdhe.ks.govMagnesium toxicity: 10 mL of 10% calcium gluconate IV over 3 minutes, furosemide, stop infusion above 9.6 mg/dL (8 mEq/L).

Facts on this page were checked against these sources on Oct 22, 2026. Guidelines change; confirm doses against current institutional protocols.