Often, there are themes related to their relationship growth. The pregnancy can either strengthen or test their bond. Maybe they have to make tough decisions together, like whether to keep the baby or not, which can bring out different sides of their personalities and how much they truly care for each other.
One common theme could be the challenges they face in high school while dealing with the pregnancy. Another might be the reactions of their friends and family. For example, their friends could be either very supportive or judgmental.
A theme could also revolve around how they balance their high school lives, such as dealing with schoolwork and extracurricular activities while preparing for a baby. This might include Clary having to drop some of her hobbies or Jace struggling to keep up with his sports while also being there for Clary during doctor's appointments and such.
It would change in many ways. They would go from being carefree high school sweethearts to having a lot of responsibilities. There would be more arguments as the stress of the situation builds up. But at the same time, they would also have deeper moments of love and understanding as they face this huge life change together. Jace might become more overprotective of Clary, and Clary would need Jace's support both emotionally and physically.
One possible plot could be that Clary and Jace face a lot of social stigma. Their friends might be shocked and some might even turn their backs on them. They would have to deal with their families' reactions, which could range from disappointment to support. They'd also have to figure out how to continue their education while preparing for a baby.
One common theme is the adjustment to their new life together. They have to deal with the changes that come with being married and expecting a baby, like setting up a home. Another theme is the reactions of their friends and family in the fanfic world. Some fics might focus on how their family supports or maybe even opposes their situation at first.
Love is a very common theme. Since they are a couple in the original series, fanfiction often explores different aspects of their love, like how they overcome obstacles to be together. Another theme is adventure. Fans like to put Jace and Clary in new adventures that are related to their world of shadowhunters.
Adventure is another theme. Given that they are Shadowhunters, many fanfictions continue their adventures, facing new demons and solving mysteries. This not only tests their relationship but also shows how they grow as individuals within the relationship. For example, in some fanfictions, they might have to go on a quest to find a lost relic, and during this journey, their bond either strengthens or is put to the test.
Alec might also play an important role as a supporting character. He could be the voice of reason for Jace, making sure he doesn't make any hasty decisions. And Luke would surely be involved. As someone who has always been like a father figure to Clary, he would offer his wisdom and support, and maybe even help them figure out the financial aspects of having a baby so young.
Love is a very common theme. Their intense, passionate love for each other is often explored in great detail. Another theme is sacrifice. They are often put in situations where they have to sacrifice something for the other.
One common theme is redemption. Jace usually has to atone for his actions that hurt Clary. For example, he might go on a quest to find a way to reverse the damage he's done.
One common theme is their forbidden love. Since they are Shadowhunters with certain rules and taboos in their world, their love often goes against some norms, which makes for a very intense and passionate story.
I'm not entirely sure if there is a specific 'Jace Superstar Clary Pregnant Fanfiction'. However, considering how creative fans can be, it wouldn't be a surprise. Some fans like to explore different life situations for their favorite characters. You could also check out Tumblr, as sometimes people share links to interesting fanfictions there.