I first talked about periods, pubic hair, penises etc with my kids when they were small, and not giving us any privacy in the bathroom.
You know how that goes. There was a time when one of my children loved putting money in the tampon machine for me in public bathrooms. I've had 3 hbs so my kids have been around laboring and birthing.
It's been ongoing, and I am happy that my boys know these things as well. When my sister told us she was pregnant, one of the children asked, 'How do you know". I was going to say she took a test, but my youngest ds piped in with, 'She probably didn't get her period when she was supposed to".
I *think* they think it's natural to talk about these things. I ask questions when it seems appropriate to, so i can see if they are understanding, or need other info. Some topics ar harder than others, of course. When my oldest was 4 and i was preg, he wanted a lot of info and really pressed me for days for more. i was trying to figure out what he wanted, so I was a little slow on facts at first. When it was clear what he was asking, i told him as appropriately as i could. He looked at me horrified, '"i cannot believe my grandparents had to do *that* 6 times". LOL