So, Ernst Haeckel was right?
If you mean Haeckel’s “ontogeny recapitulates philogeny,” then I would say no. Miscarriages due to genetic disorders are not, AFAIK, throwbacks to older, extinct forms, but rather are new changes that simply don’t work.
Aren’t many species of parasite animals that have reduced their complexity due to their reliance on other organisms for some of their bodily functions? Tapeworms and such?
Many are, I am sure, the result of non-genetic factors (e.g., health of the mother).
True, most miscarriages are probably due to non-genetic factors. The ones that are genetic in nature, however, can’t really be described as steps “backward” as they are by Haeckel, whose “ontogeny recapitulates philogeny” principle states that a developing fetus passes through all the evolutionary stages of its ancestors before finally reaching the “highest” level as a fully-developed human (a white one, btw. Haeckel also considered non-whites to be lower evolutionary stages, thus the term “mongoloidism” (Down’s syndrome) for when a fetus failed to develop beyond the asian, or mongoloid stage). In his view, birth defects (and genetic-factor miscarriages) occurred when a fetus became stuck at a lower level of development.