IV morphine
The most likely cause of the decreased variability and decreased number of fetal heart rate (FHR) accelerations is IV morphine. At equivalent doses, all narcotics (with or without added antiemetics) similarly cause a decrease in variability and a decrease in the frequency of accelerations. Penicillin, hypertension, and the presence of Group B Strep should have no effect on the fetal heart rate tracing. While abnormal FHR tracings can occur in up to 60% of women with preterm labor, the fetus in this case is 37 weeks and should not have an abnormal FHR tracing related solely to gestational age.
References:
Intrapartum Fetal Heart Rate Monitoring: Nomenclature, Interpretation, and General Management Principles. ACOG Practice Bulletin: Clinical Management Guidelines for Obstetrician-Gynecologists. Number 106, July 2009.
Young BK. Intrapartum fetal heart rate assessment. UpToDate. Updated October 17, 2011.