Those who have miscarried

I know this isn't something you may want to talk about and for those of you who have gone through something like this, I can't imagine and my heart breaks for you. I may not be alone in saying that miscarring is always a fear I have in the back of my mind. I'm currently 7w4d and I know anyone can miscarry at any time, but it seems around 6-11 weeks seems to be common for women who I've seen talk about it here. 
I guess my question is...did you have any onset symptoms or warnings before you miscarried or did it happen very suddenly and you didn't know until that day? Or did you not know until you had your ultra sound?