When should I tell employer I’m pregnant?

I was planning on waiting til week 12-13 to tell my manager but now we have a student that wants to come aug-dec when I’ll be on maternity leave and someone else will also be on maternity leave. Manager says maybe we can take the student if just one person will be off but now I’ll feel bad if I don’t tell my manager we accept the student and two of us are off 😬 I’m only 8 weeks right now! What would you do??