Final answer:
The observation that requires the nurse to notify the practitioner immediately is decreased fetal heart rate at the beginning of contractions.
Step-by-step explanation:
The observation that requires the nurse to notify the practitioner immediately is decreased fetal heart rate at the beginning of contractions. When the fetal heart rate decreases at the beginning of contractions, it could be a sign of fetal distress or cord compression, which requires immediate medical attention. This observation indicates that the fetus may not be tolerating the labor well and needs intervention.