Telling HR?!

I am currently working on a trial initiative at work that pays out a temporary stipend(bonus) for this additional work. This stipend expires after 1 year in May 2026. I am due June 2026. This stipend was set up to see if initiative is worth making permanent, and thereby having to update my job description and increase my base salary on a more permanent basis. Is it better to tell HR I'm pregnant ASAP, or after trying to negotiate updating job description/salary, knowing it may be a drawn out process? My apprehension is knowing that I'll be out on mat leave, they may decide to put a pause on initiative (and extra pay) once it expires in May and revisit job description once I return. But idk if telling them early will somehow prevent this?