Antenatal betamethasone is given to accelerate fetal lung maturation in threatened preterm delivery. Cortisol and prednisolone are ineffective for this purpose despite adequate maternal dosing because:

• A They bind maternal albumin too tightly to reach the fetus
• B Placental 11-beta-hydroxysteroid dehydrogenase type 2 converts them to inactive metabolites before fetal transfer ✓
• C The fetal liver conjugates them to glucuronides faster than they act
• D They competitively inhibit surfactant protein gene transcription

Explanation

The placenta expresses abundant 11-beta-hydroxysteroid dehydrogenase type 2, which oxidises cortisol and prednisolone to their inactive 11-keto forms, shielding the fetus from maternal glucocorticoid excess. Betamethasone and dexamethasone resist this enzyme because of fluorination at the 9-alpha position, so they reach the fetal circulation intact and induce surfactant production.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.