Board Thread:Legitimate Canon Questions/@comment-4467206-20140620014140/@comment-4091815-20140620015727

The canon answer to this question is that only a few days passed between the time they had sex and the end of Season 3(b).

In other words, not enough time has passed for such a result to be known.

The real-world answer is that sex doesn't always result in pregnancy - in fact for many people getting pregnant takes many, many attempts.