In breech delivery, head entrapment is the most serious immediate complication because the fetal head is the largest part to pass last through the birth canal. If the cervix is not fully dilated or the uterine tone is inadequate, the aftercoming head can become trapped above the maternal pelvis, leading to prolonged hypoxia and potential fetal death. Prompt recognition and skilled maneuvers or cesarean section are critical to prevent morbidity and mortality. Cord prolapse is common but less immediately life-threatening than prolonged head entrapment.

Reference: Williams Obstetrics, Cunningham F.G, 25th Edition.