The patients in this story came to the Yale Fertility Center to pursue pregnancy. They began their I.V.F. cycles full of expectation and hope. Then a surgical procedure called egg retrieval caused them excruciating pain.Some of the patients screamed out in the procedure room. Others called the clinic from home to report pain in the hours that followed. But most of the staff members who fielded the patients’ reports did not know the real reason for the pain — a nurse at the clinic was stealing fentanyl and replacing it with saline.Today, we’re sharing the first episode of “The Retrievals,” a five-part narrative series from Serial Productions and The New York Times, reported by Susan Burton, a veteran staff member at “This American Life” and author of the memoir “Empty.”