There are quite a few out there. Some focus on alternate universes where Steve Rogers experiences this rather unique situation. You can try searching on popular fanfic platforms like Archive of Our Own. Just be sure to check the tags and ratings to find the ones that suit your taste.
One common theme could be the challenges Steve Rogers faces while dealing with the pregnancy, considering his role as a superhero. Another might be the relationship dynamics between him and the other characters during this time.
The uniqueness lies in the fact that it's a complete departure from the traditional portrayal of Steve Rogers. It challenges the norm and allows for a whole new set of character developments and storylines.
Love and support are very common themes. Steve is often depicted as being extremely supportive of the OC during her pregnancy, showing his caring and protective nature.
I'm not sure about specific 'pregnant Steve Rogers fanfiction' titles. But you can try searching on fanfiction websites like Archive of Our Own. There might be a wide range of stories there.
Love and family are big themes too. The stories often show how the love between Steve and the OC grows deeper during the pregnancy. And they start to build a family unit, which might be different from a normal family because of Steve's status. It shows how they create their own version of a loving family in the face of all the challenges that come with the situation.
There's also 'Expecting with Steve'. This fanfic delves deep into the emotions of the OC while she is pregnant. It shows the support she gets from Steve and their friends in the Avengers. It has some really heart - warming moments.
One great fanfiction in this category is 'A New Beginning'. It beautifully explores the relationship between Steve Rogers and the original character as she discovers she's pregnant. It delves into their emotions, from Steve's initial shock to his growing excitement and protectiveness. The story also shows how the OC copes with the changes in her life, both physically and emotionally, while being in a relationship with a superhero like Steve.