When to tell my new boss I’m pregnant

I am a full time nanny. I leave my current position to start with a family with a newborn in one month.

I was hired for this new position before I became/understood I am pregnant. (I’m 5 weeks now)

Should I tell them now before I officially start?

It’s still early, so I haven’t told anyone but my husbands parents and my parents. I just feel like I should tell her because if I withheld this information she may be angry later on... although I was hired already? Just didn’t start yet.

What do you think?