There is no relation between the way a diaphram works and the way the morning after pill works.
DOES THE MORNING-AFTER PILL PREVENT PREGNANCY, OR CAUSE ABORTION?
It is often nature's way for cells to join and not attach to the womb without outside assistance. We do not consider this a miscarriage or even lable it an abortion. Most likely the woman never knew that she was 'almost' pregnant.
So why not assist nature and avoid an unwanted pregnancy during those first three days when we are talking about two microscopic cells that the body might have expelled naturally. This is so much better than waiting until the cells have multiplied and formed, only to be ripped apart after four months of development.