Tell it like it is, Miaou.
My version went something like: Once a woman is older, every month her womb gets ready to grow a baby. If no baby grows that month, then the thick lining of the womb comes away. It looks like blood, but it has other tissue and mucus mixed in with it. I have a science background, so I'm quite happy using terms like 'nutrients' and 'tissue' with the children, but if yours aren't used to that you may need to let them in more gently. A runny nose is not a bad analogy for the body getting rid of stuff it doesn't need any more.
I also stressed that it isn't the same kind of blood that you get from cutting your finger and that it doesn't hurt, although you sometimes get a bit of a tummy ache.
And yes, you may need to follow it up with a discussion of how the baby gets there in the first place.
I really think you need to make sure they know the basics, Miaou, especially your eldest. Saying 'wait for them to ask' is all very well, but children are liable to pick up quite subtle messages and if they get the vibe that you're not comfortable talking about it, they may not want to ask. It is a difficult path to treat -- clearly you don't want to be forcing information on them that they're not ready for, but you do need to make sure you're sending out the right signals. Mine knew the details quite early, partly because there are some very over-sexualised children in their school and I wanted to make sure they got the truth before hearing the playground version. Clearly yours are in a more protected environment, but evey so I'd be surprised if they hadn't started trying to work it all out.
Having had a baby is a great time to start -- you can say that what comes away is the stuff that has helped the baby grow in the womb, and that something similar happens to women every month. You could then follow up by saying, "if you want to know more about how babies grow, I'm happy to tell you." I've also found it helpful to say, "Some people get very silly and giggly about things like this, but that's just because they're embarrassed."
Have just posted 2 parcels to you, btw!
hth