I've made my case on several other threads, but I can give you the summary. Like many Christians in the middle ages, I don't consider an early foetus to be sufficiently developed to contain a personality (or soul, for Christians). Without a personality it is not a person (= human being). So it does not have the moral status of an extant person. Considering a person to have rights therefore does not necessarily extend that protection to the foetus, unless it can be shown that it might have such a personality that qualifies it as a person.
Your turn.
From the scientific viewpoint there is nothing in science that can substantiate a unique human being is not created at conception.
From an Orthodox Christian theological viewpoint there is nothing that can substantiate a person is not given a soul at conception.