How long can a woman survive with a 3 month old dead fetus inside her uterus?

The mother's body in almost all cases will detect that the fetus is dead, and will typically commence and complete a delivery. One case, which I suspect to be typical, documented that the mother's body retained the post-mortem fetus for 6 weeks. Other articles cite times as short as a few days, or a week.

The major factors at play seem to relate to the cause of death of the fetus as well as the mother's general health. In a healthy mother, a fetus that fails a major developmental step (usually way before 21 weeks) will miscarry on its own often before death is detected, and the mother's physiological health will not necessarily be compromised. A sad process, but a healthy one -- the body, in early term, stops what isn't going to work.

If the mother, however, is extremely ill, and/or the illness relates to the fetus, reports vary. If the mother is weakened, non-induced delivery may or may not occur, as one hopes. This case also speaks to causes for an abortion (and yes, even though the fetus is dead, the procedure is still called an abortion).