I did better than appeal to Thayers, I supplied the link where I was reading it. Section II, 22 lines down under a. refers specifically to the verse in question. Quote:" b. with the gen. of the neut. rel. pron. ou or otou (which this text has by the way - ou) gets the force of a conjuction, until, till (the time when); a. ews ou (first in Hdt. 2, 143; but after that only in later auth., as Plut. et al. [W. 296 (278) note; B. 230 sq. (199)]): foll. by the indic., Mt. 1.25 [WH br. ou]; xiii.22; Lk xiii......"
From Thayer's Lexicon (Hel0) http://www.blueletterbible.org/cgi-bin/words.pl?book=Mat&chapter=1&verse=25&strongs=2193&page=1&flag_full=1
-A8