Correct Answer: B. Prevention of RDS premature baby
Magnesium sulphate (MgSO₄) is a critical drug in obstetrics, but its role is strictly neurological and uterine, not pulmonary. The question tests understanding of MgSO₄'s mechanism of action and its evidence-based indications. MgSO₄ works as a NMDA receptor antagonist and calcium channel blocker, reducing neuronal excitability and uterine smooth muscle contractility. In severe pre-eclampsia and eclampsia, it prevents seizures by raising the seizure threshold. However, RDS (Respiratory Distress Syndrome) prevention in premature infants requires antenatal corticosteroids (betamethasone or dexamethasone), which promote fetal lung maturity by accelerating surfactant production. MgSO₄ has no direct effect on fetal lung development or surfactant synthesis. While MgSO₄ may be given to a mother at risk of preterm labour for neuroprotection of the fetus (reducing cerebral palsy risk), this is distinct from RDS prevention. Indian guidelines (FOGSI, ICMR) and Harrison's Obstetrics clearly delineate MgSO₄'s role as seizure prophylaxis and tocolysis, not respiratory support. The confusion arises because both drugs are used in preterm labour scenarios, but their mechanisms are entirely different.
Why the other options are wrong
A. Reducing contractility of uterus — This is a primary and well-established role of MgSO₄. It acts as a tocolytic agent by blocking calcium influx into smooth muscle cells, reducing uterine contractions. This is a standard indication in preterm labour management in Indian obstetric practice (FOGSI guidelines). MgSO₄ is used to delay delivery and allow time for antenatal corticosteroid administration. C. Prevention of seizures in severe pre-eclampsia — This is a gold-standard indication for MgSO₄ in India and globally. It raises the seizure threshold by antagonizing NMDA receptors and blocking calcium channels in the CNS. MgSO₄ is the first-line anticonvulsant in severe pre-eclampsia (BP ≥160/110 mmHg with symptoms) per Indian guidelines, superior to phenytoin or diazepam in preventing eclamptic seizures. D. Prevention of recurrent seizures in eclampsia — Once eclampsia (seizure) has occurred, MgSO₄ is the definitive maintenance therapy to prevent further seizures. It is given as a loading dose (4–5 g IV) followed by maintenance infusion (1 g/hour) until 12–24 hours postpartum. This is a cornerstone of eclampsia management in Indian obstetric units and reduces maternal mortality significantly.
High-Yield Facts
- MgSO₄ mechanism: NMDA receptor antagonist + calcium channel blocker → reduces neuronal excitability and smooth muscle contractility.
- RDS prevention: Requires antenatal corticosteroids (betamethasone 12 mg IM × 2 doses, 24 hours apart), NOT MgSO₄; acts by promoting fetal surfactant synthesis.
- MgSO₄ seizure prophylaxis: Indicated in severe pre-eclampsia (BP ≥160/110 mmHg + symptoms) and all cases of eclampsia; superior to phenytoin (Dilantin).
- MgSO₄ tocolysis: Delays preterm labour by reducing uterine contractility; allows time for corticosteroid administration and maternal transfer to tertiary centre.
- Neuroprotection: MgSO₄ given to mothers at risk of preterm delivery (<32 weeks) reduces cerebral palsy risk in surviving neonates, but does NOT prevent RDS.
- Monitoring: Check serum magnesium levels (therapeutic 4–7 mEq/L); loss of patellar reflex signals toxicity; ensure urine output >100 mL/4 hours before each dose.
Mnemonics
MgSO₄ Roles (NUTS) Neuro (seizure prevention in pre-eclampsia/eclampsia) | Uterine (tocolysis, reduces contractility) | Toxicity (monitor reflexes, urine output) | Surfactant (NOT involved — corticosteroids do this) RDS Prevention = STEROIDS, not MgSO₄ Steroid (betamethasone/dexamethasone) → Timing (24 hours before delivery) → Effect (surfactant synthesis) → Respiratory (lung maturity). MgSO₄ has NO role here.
NBE Trap
NBE pairs MgSO₄ with preterm labour scenarios (where both MgSO₄ and corticosteroids are used) to trap students into conflating tocolysis/neuroprotection with RDS prevention. The trap: "MgSO₄ is used in preterm labour, so it must prevent RDS" — but it doesn't.
Clinical Pearl
In Indian obstetric units, a mother with severe pre-eclampsia at 28 weeks receives MgSO₄ for seizure prevention AND betamethasone for fetal lung maturity — these are complementary, not redundant. Confusing their roles is a common NEET PG trap. MgSO₄ protects the mother's brain; corticosteroids protect the baby's lungs.
_Reference: Harrison's Obstetrics & Gynaecology Ch. 6 (Pre-eclampsia/Eclampsia); FOGSI Guidelines on Management of Hypertension in Pregnancy; DC Dutta's Textbook of Obstetrics Ch. 18_