The factor that differentiates a prolonged deceleration from bradycardia is:

Comprehensive and Detailed Explanation From Exact Extract–Based NCC C-EFM References:

NICHD/NCC definitions:

Prolonged deceleration: decrease in FHR ≥15 bpm lasting 2 to 10 minutes

Bradycardia: baseline FHR <110 bpm lasting ≥10 minutes

The differentiating factor is duration, not rate and not contraction relationship.

Before 10 minutes → prolonged deceleration

At or beyond 10 minutes → new baseline → bradycardia

Thus, the factor that differentiates the two is length of time it lasts.