
Birth control is apparently one of those things that slips his scattered mind, and the next thing you know you've got a little kid running around thinking bow ties are cool and eating seafood mixed with dairy desserts.
In other words, you wind up with The Boy. (Shhh, don't tell Uncle Walter! -- The Wife)

P.S. That's just a joke. If The Wife were going to have an affair with a Doctor, it would be with Ten, not Eleven. No offense to Matt Smith.
The Doctor's Secret Child -- Kate Welsh