Final answer:
Uterine contractions, fetal movement, and hormonal changes cause the descent of the fetus through the pelvic canal during childbirth.
Step-by-step explanation:
The descent of the fetus through the pelvic canal is primarily caused by uterine contractions. During labor, the contractions push the baby's head (most often) against the cervix, causing the cervical canal to dilate and become wider. This dilation allows the baby's head to fit through the cervical canal.
Additionally, fetal movement and hormonal changes also play a role in the descent of the fetus. The stretching forces from the fetus and the drop in progesterone levels lead to increasing uterine irritability and prompt labor. Fetal movement and stretching of the myometrium and cervix are stimulants for uterine contractions.
The placental shift is not a direct cause of the descent of the fetus through the pelvic canal, but it does occur after the baby has been born during the last stage of childbirth.