Definitely not a common theme. The Naruto fandom has a vast range of fanfic topics. For example, there are many that focus on Naruto's training, his battles, or his friendships. Since the original Naruto story has a clear family tree where Kushina is Naruto's mother, this pregnancy - related fanfic idea is quite out of the ordinary. It also might make some fans uncomfortable as it goes against the established family dynamic. However, in the very large and diverse world of fanfiction, every once in a while, there might be some out - there ideas like this, but they are far from the norm.