We've talked "forever" about this stuff...pregnancy came up when I was pregnant with her brother, she's seen labor and birth, she's seen breasts and pubic hair, which started questions about puberty, she's seen menstrual pads and me charting, which started conversations about cycles and birth control. It's all been very gradual and we'll often re-visit topics when she wants more info.
We have a copy of What's Happening to My Body Book for Girls which talks frankly about all kinds of stuff. She's said she doesn't want to read it yet, but she knows it's there for her. In our conversations, I just take my cues from her--she lets me know when she doesn't want me to go any further with the info. At 8, she wouldn't have wanted to know about charting temperatures and ovulation, yet at 10 she likes the idea that her period doesn't need to be mysterious--that she can keep track of things. I'm sure she's not going to be charting herself any time soon, but she's open to the info now.
I don't think you can go wrong by giving info that she's comfortable with, and leaving the conversation open when she says she's heard enough.
Single Mom to 3 (12, 17 & 21) and .