Our Catholic brethren assure us that Human Life begins at conception and comes from the union of sperm and egg but this has not always been a foregone conclusion in that Church. St Thomas Aquinas discusses the issue at length in a section of the Summa Theologica: Of The Production of Man from Man As to The Soul.

The material here is not for the timid. But, as nearly as Wonks Anonymous can figure this one out, the fetus starts with a vegetative soul, capable of drawing nourishment and growing. This is replaced at some point by an animal, soul which can move and sense things. Finally, at some point not defined, God steps in and endows the fetus with a human intellectual soul.

In the Middle Ages, tradition appears to have assigned these three souls to the three trimesters of pregnancy. Is this the eternal truth or is the more recent doctrine more true?