Well, it's not common at all. The Harry Potter series has a vast range of fanfiction, but this idea is quite extreme. The original story is mainly about Harry's fight against Voldemort, friendship, and the magic world's mysteries. Such a pregnancy plot between them seems very far - fetched and doesn't really align with what most fans expect from fanfiction. It might be a very niche and perhaps even a bit of a shock - value kind of concept that only a small number of writers might explore.
It's not a common trope in the general Harry Potter fanfiction community. Most fanfiction tends to focus on more traditional relationships and storylines within the wizarding world. This kind of plot is rather outlandish and goes against the typical character dynamics and the overall tone of the original series.