Development stopped long before there was any recognizable embryo. Miscarriage cannot occur until progesterone levels fall. Continued progesterone secretion will delay miscarriage, eventhough there may be no viable embryo. Giving progesterone cannot make bad development normal. An embryo develops inside a shielded and insulated micro- environment that is very well protected against external factors.
Miscarriages are a natural mechanism that protects against abnormal embryonic development.
Chromosome problems seem to lead the list. When the problem involves a large chromosomes, having a lot of genetic material, development stops before there is any recognizable embryo. The smaller the chromosome that is abnormal, the further development may progress before halting. The structure that looks like a diamond ring is a small embryonic remnant atop a thin walled, distended yolk sac. There is no heart beat.