Anna, not sure what your definition of an embryo is but there's no fertalization going on when one has an IUD.
I researched IUD's for almost a year (online, talking to people who had them in the past and currently, and talking to my OB/GYN) before deciding it was the safest, healthiest for of BC I could use. The IUD's of today are much improved over what they were several decades ago. And it's not the IUD being in your babies head that one has to worry about... it's the baby growing in your tubes that is the real danger. That however outweighed the other alternatives.
Bottom line... IMO... if it's not a life (and of course we believe, or maybe it's just me, that life begins at conception) then you're not killing. If preventing pregnancy with BC is abortion then so is abstaining... if you abstain then you're also preventing conception and playing God aren't you??
We're expect to be in some sort of control of our lives... we don't let the wind decide what we're going to do each day (unless we fly in hotair balloons). I think the reason that this topic isn't directly discussed in SOP is b/c it's a personal decision between husband and wife as to what is best for their family.
Ooopsss... have to fix dinner and rush off to prayer meeting!
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"Decide what you want, decide what you are willing to exchange for it. Establish your priorities and go to work." ~H. L. Hunt