It's very common to miscarry at 12 weeks pregnancy if the placenta fails to implant - this marks the phase between the embryonic and foetal phases.
A miscarriage may occur at any time for many reasons, however. For example, something as simple as a urinary tract infection (cystitis) may induce a miscarriage, or early labour.
An early stage miscarriage is likely to be simpler than a later state one, but there may be complications requiring medical intervention either way.
It can be like a period, or it may involve going through labour.
It can be emotionally devastating regardless of when, why, or how it happens.
RE: A great many people think they are thinking..