Betamethasone is administered intramuscularly to a pregnant woman at 30 weeks gestation with threatened preterm labour. The reason betamethasone is the preferred corticosteroid for fetal lung maturation is:

• A It has the highest mineralocorticoid potency among synthetic steroids
• B It readily crosses the placenta, has high glucocorticoid potency and a long duration of action with minimal mineralocorticoid activity ✓
• C It is inactivated by placental 11 beta-hydroxysteroid dehydrogenase type 2, protecting the fetus from excess exposure
• D It acts directly on fetal type II pneumocytes without crossing the placenta

Explanation

Antenatal betamethasone (or dexamethasone) works because these fluorinated steroids cross the placenta efficiently, possess very high glucocorticoid potency, act for 24 hours or longer allowing two-dose regimens, and have negligible mineralocorticoid activity. They induce surfactant lipoprotein production by fetal type II pneumocytes and accelerate lung maturation. Option C describes the normal placental barrier enzyme, which these steroids must escape rather than rely on. Hydrocortisone is largely inactivated by the placenta, which is precisely why it is not used for this indication.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.