I strongly doubt that their child was conceived during Turn Coat but I wouldn't be surprised to discover it was conceived due to it. They have unprotected coitus on DR during the events, it turns into a pregnancy "scare", then Georgia and Billy realize that they do actually want a child. I know a few people, including my parents, who intentionally conceive after similar circumstances.
Or for what ever reason, the actual conception was delayed. Don't ask me how, just a thought. However what I find strange, in 12 Months, Will is always at the castle, early morning, fixing breakfast, late at night, working out, taking notes. Will has changed since the Battle for Chicago, even Harry mentions in 12 Months that Will frightens him, however he can't seem to put his finger on it. The point I am getting to this means Will is away from his family a lot! Georgia is a capable woman, but I know of few mothers who would be happy about her husband gone a lot, especially with a young child at home. What about Will's work or his business? That too is being neglected, yet Will never mentions, wife,child, or business to Harry, positive or negative. Very strange, the Pack shows up a couple of times, but while Harry mentions them showing up, he doesn't name any of them. He used to do that.