A 28-year-old primigravida at 34 weeks of gestation presents with a history of transfusion with unmatched blood during a motor vehicle accident 10 years ago. She is Rh-negative and her partner is Rh-positive. Antenatal screening shows an indirect Coombs test (ICT) positive at 1:16 dilution. Which investigation is most appropriate to assess the severity of fetal hemolytic disease and guide management?