Benzylpenicillin IV 3g initially, then 1.5g 4 hourly until delivery

If true penicillin / beta-lactam allergy:

Teicoplanin IV 12mg/kg (dosing table here) over 3-5 minutes as a slow IV bolus or over 30 minutes by IV infusion every 12 hours until delivery - use most recent body weight and round each dose to nearest 100 mg (max 800 mg).