Definitely not appropriate. Fanfiction is supposed to be a positive way to explore the world and characters of a particular story. Such a description violates the respect that should be given to the characters and can also make many readers uncomfortable. It goes against the principles of good - quality fan - created content.
If this is just a fictional story, it's important to remember it's not real. But if it makes you uncomfortable, you could stop reading or find a different story.
One great Naruto female Haku fanfiction could focus on her past before she met Zabuza. It could explore her life as an orphan, her struggle to survive, and how she discovered her unique ice jutsu abilities. This would give readers a deeper understanding of her character and what made her so loyal to Zabuza later on.
Most official platforms would not allow such inappropriate fanfiction. Fanfiction should be about creativity within the boundaries of respect for the source material and its characters. Instead of seeking such a fanfiction, you could search for ones that depict their relationship in a more platonic or mentor - mentee style, which are much more common and in line with the spirit of the 'Harry Potter' universe.
Well, in some fanfiction ideas, Naruto might use some powerful jutsu. For example, he could discover a new form of the shadow clone jutsu that allows him to transfer life force. He clones himself multiple times and combines their life energy to bring Haku back. Another way could be through some kind of ancient forbidden jutsu he stumbles upon. Maybe it's a jutsu that requires a great sacrifice on his part, like giving up some of his own chakra permanently.
I'm not sure if there's a specific and widely - known 'Naruto x Haku pregnant fanfiction'. But it's very likely to exist given the creativity of fanfiction writers. They often take existing characters and put them in unexpected situations. A story about Haku being pregnant could involve all kinds of new plotlines, like dealing with the reactions of other characters in the Naruto world, or how Haku would handle the physical and emotional changes. It might also be a way to introduce new elements to the Naruto universe that were not present in the original series.
For many, it would be disturbing. Reading or hearing about such a situation can be difficult because it challenges the basic understanding of trust and commitment in a relationship. It might also make people think about the fragility of relationships and how easily they can be broken by such extreme actions.
One consequence could be that Naruto is seen as a hero by some, but also as a rule - breaker by others. The ninja world has rules about bringing back the dead, and Naruto's actions might stir up controversy. Some might fear that this new ability of his could be misused. Haku, on the other hand, would have to adjust to a world that has changed since his death. He might face challenges in reconnecting with his old friends or finding his place in the new ninja world.
In a fanfiction, Naruto and Female Haku could form an unlikely friendship. Maybe Haku is initially distrustful of Naruto because of his connection to the Hidden Leaf Village. But as they spend time together, they realize they have more in common than they thought. They could learn from each other's skills and experiences, and perhaps even work towards a common goal like bringing peace between different ninja clans.
In fanfiction, Naruto and Haku can have a very unique relationship. Naruto, with his big heart, could potentially befriend Haku in ways that the original story didn't fully explore. They might share adventures and help each other grow as characters.