Telling my job I’m pregnant

i am almost 10 weeks and my boss is having surgery and is going to be out for 6 weeks in august. By the time she returns I will be almost half way through my pregnancy. Should i tell her before she goes out? Also I’m trying to get a promotion with a big pay raise before I tell her I’m pregnant because I’m nervous my news may affect the position. Should i say something before she goes out and officially gives me the new position or wait till she gets back? I’m at a loss here. I don’t want to seem like i was trying to hide it from her for half of my pregnancy.