The relationship between IgM confirmed-rubella infection (i.e., using a blood antibody test for rubella given if requested by the attending physician) during pregnancy and the development of congenital malformations in the offspring in the first year of life can most appropriately be studied with which of the following designs? (choose one best answer) Randomized trial Cross-sectional study O Retrospective cohort study O Prospective cohort study Case-control study