Final answer:
Hypertonic contractions caused by oxytocin stimulation can reduce placental blood flow, which is harmful to the fetus as it compromises its oxygen and nutrient supply.
Step-by-step explanation:
The side effect of oxytocin stimulation in childbirth can lead to hypertonic contractions. These are overly strong uterine contractions that can potentially be harmful because they may cause a reduction of placental blood flow. This is detrimental to the fetus as it relies on the placenta for oxygen and nutrient exchange. If the blood flow is reduced, it compromises the oxygen and nutrient supply to the fetus, which can lead to fetal distress and other complications.