Work and pregnant

I'm planning to tell my boss at the end of this month about my pregnancy.

Just last week we had a small discussion about family and marriage and when the right time is. She argued that even at your 40s is the best time. She then asked me 3 times if I'm planning to get married with kids, I said sure I got already my fiance.

Later a co-worker tells me that my boss has this "opinion" because she only cares about her business and the longer you hold off kids the better for her.

I'm hesitant to tell her now because of this. Even though under law she can't fire me after knowing, I'm assuming she'll cut my hours drastically. She has done this when I've been officially sick btw.

When I tell her, do I need a written form of sorts to confirm she knows? How do you go about this?