The status of embryo
An embryo is not only the part of the body of a woman. As biological structure it is not similar to any organ of the body because it is the human being that grows inside another human body.
The human embryo has special ontological status – it is "potential person." Its nature is usually biological and in social plan it is just an object of care in normal conditions excluding the case of abortion.
Its nature is the development, formation of biological individuality. It has all chances to form the unique characteristics of the soul and the character of the future person.
Any stage of the development of the embryo can't be named "the formation of the human being" as all of them make system that forms the baby. And day by day the process of formation of the embryo into the person comes to its end.
The special ontological status of embryo also defines its special moral status. On any stage of its development embryo has the human dignity. The foetus gets special moral status because it participates in every-day moral relations of people making its influence on them.
In fact the embryo gets the moral status when mother first realises that she is going to be the mother, at the moment when mother decides to make abortion or not, etc. It gets the moral status when it becomes the object of moral attitude.