Complex sentence: Hilgers began research in 1976 which led to the development of the Creighton Model FertilityCare System, which was first fully described (as the Creighton Model) in 1980.
Split into simpler sentences: Hilgers began research in 1976 which led to the development of the Creighton Model FertilityCare System. The system was first fully described (as the Creighton Model) in 1980.