A 26-year-old primigravida at 41+3 weeks with a Bishop score of 3 is planned for induction of labour. Cervical ripening is performed with a 25 mcg intravaginal misoprostol every 4 hours. After 3 doses, uterine hyperstimulation occurs (>5 contractions in 10 minutes lasting >90 seconds). Which receptor mechanism underpins misoprostol's efficacy for cervical ripening and which tocolytic should be given for hyperstimulation?

• A Oxytocin receptor partial agonism; atosiban IV is the tocolytic of choice
• B FP prostaglandin receptor activation causing cervical softening; indomethacin should be given
• C EP2/EP3 prostaglandin receptor agonism; terbutaline 0.25 mg SC is first-line for hyperstimulation ✓
• D EP1 receptor activation causing myometrial relaxation; nifedipine 10 mg sublingual is preferred

Explanation

Misoprostol is a synthetic prostaglandin E1 analogue that acts on EP2 and EP3 receptors in the cervix and myometrium, promoting cervical softening (remodelling via collagenase activation) and uterine contractions. For uterine hyperstimulation/tachysystole with or without fetal heart rate changes, first-line management is subcutaneous terbutaline 0.25 mg (a beta-2 agonist) or IV tocolysis with terbutaline, which provides rapid onset of action. Unlike oxytocin-induced tachysystole, there is no reversal agent for misoprostol, reinforcing the importance of prophylactic dosing protocols.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.