In my humble opinion, the whole issue revolves around one basic question: Does an embryo possess vinnana? If it does posses vinnana, then of course, disposing of it is wrong, and if it doesn't, then doing so can't be defined as killing.
I take the position that an embryo cannot possess vinnana prior to the differentiation of tissue at 4 weeks. I am of this opinion because vinnana arises in dependence on the six fold sense base, and prior to the differentiation of tissue, there is no six fold sense base, so how could there be vinnana present in an embryo at that stage?
After 4 weeks you have a crude nervous system beginning to form, so I think you could have a very basic kind of vinnana arising, so destroying the embryo at that point would be killing.
Again, that's just my opinion, and I am looking forward to other arguments either way.
The non-doing of any evil,
The performance of what's skillful,
The cleansing of one's own mind:
This is the Buddhas' teaching.