John Chrysostom — as excerpted in the Catena Aurea on Luke 1:36-38

“(49 in Gen.) Seeing that his previous words had overcome the mind of the virgin, the angel drops his discourse to a humbler subject, persuading her by reference to sensible things. Hence he says, And, behold, Elisabeth thy cousin, &c. Mark the discretion of Gabriel; he did not remind her of Sarah, or Rebecca, or Rachel, because they were examples of ancient times, but he brings forward a recent event, that he might the more forcibly strike her mind. For this reason also he noticed the age, saying, She also hath conceived a son in her old age; and the natural infirmity also. As it follows, And this is the sixth month with her who was called barren. For not immediately at the beginning of Elisabeth’s conception did he make this announcement, but after the space of six months, that the swelling of her womb might confirm its truth.”