Good Morning. 

"From a moral point of view this is certain: even if a doubt existed concerning whether the fruit of conception is already a human person, it is objectively a grave sin to dare to risk murder. "The one who will be a man is already one."[20] (declaration on procured abortion, (CDF)

"The Magisterium has not expressly committed itself to an affirmation of a philosophical nature, but it constantly reaffirms the moral condemnation of any kind of procured abortion. This teaching has not been changed and is unchangeable." Donum Vitae, CDF

I couldn't locate the bit I was thinking of, but these two excerpts show the reluctance to pronounce personhood/the soul to be present at conception.  And I couldn't find anything declaring that it was. 

The following link goes to a discussion of the variability of doctrine over time. 
http://www.religiousconsultation.org/News_Tracker/moderate_RC_position_on_contraception_abortion.htm