Oneofthewomen is correct about the three conditions necessary for sin to be mortal. But look at what the Catechism of the Catholic Church says about full knowledge (emphasis added):
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense.
But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
We need to be very careful here. It would be a mistake to say that, until someone reads it explicitly in the Catechism that a specific act is wrong, they don’t
really have full knowledge. We all have the natural law written on our heart, so we cannot feign ignorance just because it is convenient.
Nonetheless, there really are situations where someone could be unintentionally ignorant (perhaps every Catholic they have ever known–including priests–has told them that using artificial contraception is okay). I think that, if a person is in doubt, going to Confession is never a bad thing. If the person suffers from scrupulosity, that would be another thing to take into consideration. But priests know how to handle that sort of stuff.