User blog comment:Il Lupo/New Assassin's Creed Game/@comment-1153722-20110502171431

Okay, so I have been researching the Codex... and found this:

Page #24: "Someday I will have a child – such is the way of our Order." As of that point, he didn't have a child. Altaïr lived in Masyaf since Bloodlines up to that point and made all of the Codex inventions, including his armor and excluding the hidden gun. Page #25: Shows a drawing of Maria in her Templar clothes.

So there was in fact a period of unknown activities between Bloodlines and the conceiving of his child. This game may very well take place in that period.