How can an acorn be an oak tree? 

All adults are really just "over-grown embryos". Human life starts at the moment of conception - fact. From the moment the sperm fuses with the egg life exists in all its wonderful potential to become a fully developed human person - fact. 

These scientific facts were not known until the 20th century. Prior to that theologians and the Church were always opposed to attempts to prevent conception or ending pregnancy. They didn't understand biology so got tangled up unnecessarily in issues of "ensoulment" and prior to this considered abortions to be contraception. Even so, they regarded any attempt to kill life as gravely sinful. 

Today we seem to be reverting back to this. We do all we can to prevent to prevent conception - and if it should occur against our wishes, we kill the life that results.