How a medical ethicist thinks this is all ''straightforward'' is a tad frightening.
It wasn't my intention to suggest the whole argument about the moral status of an embryo or foetus is straightforward. Actually quite the opposite - I was railing against those that come out with the simplistic (and to my mind untenable) argument that "life begins as conception". Coming out with such a glib and simplistic claim ends up with you tying yourself in biological and metaphysical knots as I indicate. The straightforward part is to strip all that away by recognising that an argument that "life begins as conception" is logically untenable. But don't just take my word for it - Mary Warnock spent two years looking into this for her report that lead to the Human Fertilisation and Embryology Act also took this view. And, this also from Jean Porter:
"until the possibility of twinning is past, we cannot say that this presently existing [embryo] is definitely identical with one specific human being which will exist in the future"
None of this provides an answer as to when life does begin, and I think that is an incredible complicated question that doesn't lend itself to a simple answer and likely we cannot ascribe any single point that defines when life begins, but we can see a gradual development of important features that are critical for personhood and we may therefore consider that the rights of the developing embryo or foetus also develop over time. Actually trying to determine some arbitrary point when life begins seems to be missing the point - the key issues seem to me to be:
1. What is the moral status of the developing embryo and foetus at various stages of development
2. What rights does the developing embryo and foetus at various stages of development
3. How do those rights conflict with the rights of the mother and how do we resolve those conflicts