How long can a 21-week-old baby stay dead in its mother's womb?
The clinical aspects are the most scant (I'll explain why later), and one of the papers I reviewed for this answer was dated 1898. Modern papers are harder to come by and, when you do find them, they carefully, delicately focus on one or another aspect of fetal mortality -- as opposed to addressing the matter generally. That said, here is the little I can tell you.
The exact amount of time is indeterminate. 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).
Typically, most elective abortions occur very early in the first trimester, and involve procedures that put the mother at very little risk. Late-term abortions are another matter, and many doctors are hesitant to perform a late-term elective abortion -- or flat-out won't do it.
Therapeutic abortions, however, are another matter. Therapeutic abortions are those abortions that are dictated out of medical necessity, where the determination is made that the mother's life is endangered if the pregnancy continues. There are lots and lots of reasons for a therapeutic abortion in later pregnancy: One of these is a post-mortem fetus.
Very few clinical trials regarding this procedure are on record. Part of the reason for this is that medical technology is moving right along, and methods change, the old giving way to the new. That 1898 paper was historically interesting, but clinically useless.
The other factors -- the political and real-world ones -- are the other reason. Over the past decade, pressure against Pro-Choice groups has been increasing, culminating in the "2003 Partial Birth Abortion Ban," sponsored by President G.W. Bush. Oddly, the bill, which is now in law, neither differentiates between a therapeutic abortion and an elective abortion, nor does it draw any line between an abortion performed on a live -- versus a dead -- fetus.
The result is that Dilation and Extraction (D&X), often the procedure of choice for delivering a post-mortem fetus, is now illegal. In addition, many hospital boards have become extremely reluctant to approve Dilation and Evacuation (D&E) -- a method similar to D&X.
In addition to this, many Catholic schools and hospitals have followed suit and have placed an ecclesiastical ban on performing D&E as well as D&X abortions.
The result is that most doctors who can perform a D&X are over 50 years old. The majority of younger doctors don't have much experience in this, and are understandably reluctant to take on the legal risks of performing a procedure in which they have insufficient experience.
So now we're seeing a lot of natural deliveries of post-mortem fetuses, partially because -- in earlier days -- they would have been therapeutically aborted. As such, we don't know how long a dead fetus can, in fact, remain in the mother's body; in the past, we wouldn't have let it go so long.
Generally speaking, with a healthy mother and a post-mortem fetus, it's clinically wise to abort the fetus. Complications with the mother's health can, of course, change this decision (anemia is one reason). And various blood dyscrasias tend to become more and more likely as time passes, notably Disseminated Intravascular Coagulation (DIC -- the med-student's mnemonic for "Death Is Coming") and thrombocytopenia. The risk of bacterial infection also increases.
One might be inclined to think that therapeutic abortion is also risky -- and it is -- but not as much.
"A review of 300 second-trimester abortions published in 2002 in the American Journal of Obstetrics & Gynecology found that 29 percent of women who went through labor and delivery had complications, compared with just 4 percent of those who had D&Es."
Ms. Magazine -- Martha Mendoza -- Essay, Summer of 2004.
Ms. Mendoza also cites that "abortion is not readily available in 86% of the counties in the US." Ibid
One aspect of this issue that is frequently neglected is the mother's psychological well-being. A miscarriage or stillbirth is a deeply traumatic event. Worse, however, are the twin dangers of either carrying a dead fetus to term, on the one hand, or fighting one's way through a medico-legal bureaucracy, trying to get this done.
Either way, the mother's mental well-being is surely going to be challenged.
So, in summary, 6 or more weeks is possible; and 3 days is also a possibility. The longer this continues, the greater the likelihood of clinical dangers to the mother. However, considering the physiological and psychological dangers the mother is facing, these scanty estimates can change, quite literally, in a heartbeat.