[hocndoc]

All but the first of your list is established at fertilization. What you would do if you were to establish your list as law is create a discrimination between those embryos within women and those which begin outside of the body. You would create/perpetuate 2 classes of humans - those created by external manipulation of gametes and those conceived within women.
Louise Brown and all the IVF babies are no less human than I am.

Twinning is one of the natural possibilities of humans at the earliest stages of development. Simply because we cannot at this time detect which zygotes are destined to twin does not make them any less individuals of the human species.

Once we begin to discriminate which humans may be killed at the whim of others, the powerful may kill any they choose to discriminate against.