A 32-year-old multiparous woman (Rh-negative, 2 previous vaginal deliveries) presents at 18 weeks gestation with vaginal bleeding. Ultrasound confirms a viable intrauterine pregnancy. Her indirect Coombs test is negative. She has not received any anti-D prophylaxis in this pregnancy. On examination, there is mild vaginal bleeding but the cervix is closed. Fetal heart rate is normal. What is the most appropriate immediate management regarding Rh isoimmunisation?