How long does a natural miscarriage take to start?

I've heard many stories of women who were pregnant, didn't even realise it, taking a pregnancy test coming up negative, then having an ultrasound showing a pregnancy that stopped growing a long time ago whether it be a month two or more. Does anyone else have experience with this type of miscarriage that doesn't pass immediately.

Sometimes you can miscarry and you wont know it because it happens when your period is due and you pass it off as a period. I learned in school that some women will miscarry but the fertilized egg/baby does not actually come out. It can remain in there. If you are aware of a miscarriage it is important to make sure there is nothing left in there (placenta, baby, etc) which is why if a miscarriage happens around the time of the period women may not know that the "contents" of the deceased baby is still inside. But you can still get pregnant with that still inside. It is just important to get those remains out.

I happened across this website: http://waitingtomc.blogspot.com/It's a blog from a woman that was told she was going to miscarry and had to wait weeks for it to happen. I don't have any personal experience with it, but what she says is so sad!