With all due respect, the loss rate for T21 is actually rather greater after 14 weeks. Most of the time T21 pregnancies that have dismal outcomes end because of either hydrops or cardiac malformations. These are things that do not become a serious problem until the fetus gets to a certain size, generally sometime after 13 weeks. In the case of hydrops, death is caused by the inability of the system to dispose of wastes; in these cases fetal death normally happens between 16-24 weeks.
A 2004 article in the journal Fetal Medicine related these statistics:
The rates of spontaneous fetal death in trisomy 21 between 12 weeks (when NT screening is carried out) and 40 weeks is about 30% and between 16 weeks (when second trimester maternal serum biochemical testing is carried out) and 40 weeks is about 20%.
FWIW, I lost three pregnancies due to trisomies, 2 of the 3 in the early second trimester. Trisomies in live fetuses are normally not detected until about 12 weeks, but once you see them you are looking about about a 1 in 5 chance that the baby will not make it to term if it is T21 (Down), or about a 1 in 2.2 chance if it is T13 or T18, which have an 80% mortality rate after prenatal diagnosis.
So yes, more trisomy losses happen in the first trimester, but the number that occur in the second is by no means negligible. For a woman who is 45 and has a 1 in 21 likelihood of a trisomy conception, the rate of loss for that reason is higher than in a younger woman.
As to missed abortions, the rate is fairly high; something like 10% of miscarriages do not complete without medical intervention. If that's what you are dealing with at 16 weeks, most of the time you won't know until a doctor checks.
FTR, the technical rule for deeming it a "spontaneous abortion" (i.e. miscarriage) vs. a stillbirth is 20 weeks gestation at demise. If the fetus is measuring smaller then the apparent age at demise is what will be used to determine what it is referred to as. Most people will are aware of the pregnancy will consider it a death, especially when it happens any time after 12 weeks. The distinction is important in some religions, and in some cases employers and/or insurance policies will decide that it makes a difference in terms of how much leave you are allowed. That was true in my case: the temporary disability policy that I have will let you have leave after delivery if it is termed a stillbirth, but not if it is counted as a miscarriage.