When looking at the Greek text of St. Matthew's gospel, in chapter 1 verse 25, the English reads, "but [St. Joseph] knew her not until she had borne a son." The Greek for until in this case is έως. I've been told that έως can mean up to and beyond a point, so the translated English could read, "but [St. Joseph] knew her not until, and after, she had borne a son."