St. Jerome - on "Since Mary his mother was betrothed" Matthew 2:18

 from St. Jerome Commentary on Matthew (Fathers of the Church vol. 117, CUA Press) 

translated by Thomas P. Scheck, 2008 ISBN = 9780813201171

1.18 "Since Mary his mother was betrothed"

Why is he conceived not from a simple virgin but from one betrothed?

FIRST, so that the origin of Mary would be shown through the genealogy of Joseph;

SECOND, to prevent her from being stoned by the Jews as an adultress; 

THIRD, so that when she fled to Egypt she might have solace;

The Martyr Ignatius (in his TO THE EPHESIANS 19.1) added even a fourth reason, saying

FOURTH, in order that his birth would be concealed from the devil,

so long as he thought that the babe was not born of a virgin but not of a wife.

[Scheck adds in Footnote on page 62 of the translated COMM. that

According to J. Lightfoot, Jerome's knowledge of Ignatius was secondhand, not direct,

and derived from Origen.