Final answer:
Flexion ensures that the smallest anterior-posterior diameter of the fetal head enters the pelvis.
Step-by-step explanation:
The factor that ensures that the smallest anterior-posterior diameter of the fetal head enters the pelvis is flexion. Flexion is the movement of the baby's head towards its chest, which allows the smallest part of the head to enter the pelvis first. This position, known as occiput anterior vertex presentation, is associated with the greatest ease of vaginal birth.