Sorry Le Ann, I didn't see your post before I added that last sentence. I'm still not sure if garters have a vascular connection to the developing eggs yet, but if they do, and they are truly ovoviviparous animals, then that connection is needed or the eggs will not be able to get the necessary gas exchange needed if they are removed from the mother prematurely. I think that's what happens when you find fully and normally developed young being born dead. I think what happens there is that the connection needed for gas exchange happens prematurely and the young basically suffocate before they are able to resume breathing on their own. If we could test the dead young in the lab and find that their blood is high in Co2 and low oxygen, that would all but confirm the hypothesis.

Removing them from the mother would be like ripping the shell and shell membrane off of a chicken egg and expecting the embryo to live. You would essentially be doing the same thing if you placed a whole chicken egg in an oxygen depleted atmosphere. The young inside the egg would suffocate.