A 38-year-old woman, gravida 3, para 1, is admitted to the hospital at 33 weeks’ gestation because of temperatures to 38.9C (102F) and back pain for 2 days. Examination shows costovertebral angel tenderness. She has had mild contractions for 4 hours. Urinalysis is consistent with a urinary tract infection. Intravenous ampicillin therapy is started for suspected pyelonephritis. The fetal heart rate pattern is shown. Which of the following is the most likely explanation for this pattern?