Obj. 3: Further, a child has a rational soul for some time before birth, just as well as after. But it does not appear that an angel is appointed to guard a child before its birth, for they are not then admitted to the sacraments of the Church.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic birth time
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“2: Further, men are guarded by angels in as far as angels enlighten and instruct them. But children are not capable of instruction as soon as they are born, for they have not the use of reason. Therefore angels are not appointed to guard children as soon as they are born.
Obj. 3: Further, a child has a rational soul for some time before birth, just as well as after. But it does not appear that an angel is appointed to guard a child before its birth, for they are not then admitted to the sacraments of the Church. Therefore angels are not appointed to guard men from the moment of their birth.
On the contrary, Jerome says (vide A. 4) that "each soul has an angel appointed to guard it from its birth."
I answer that, as Origen observes (Tract. v, super Matt.) there are two opinions on this matter.” source