Final answer:
The number one concern if the fetus starts engagement is shoulder dystocia.
Step-by-step explanation:
The number one concern if the fetus starts engagement is c) Shoulder dystocia.
Shoulder dystocia occurs when the baby's head is delivered, but their shoulders get stuck behind the mother's pubic bone. This can cause complications and endanger the baby's health. It requires quick and careful management by medical professionals to resolve the situation.