One common theme is the change in relationships. For example, how the character's partner or friends react to the pregnancy. Another theme is the anticipation and fear of the unknown. Since the Undertale world has its own unique elements, the pregnancy might bring new challenges and uncertainties. Also, the theme of protection often comes up, with the character and those around them being protective of the unborn.
In 'Undertale Pregnant Sans' fanfiction, a frequently seen theme is the relationship dynamics. How Sans' relationships, whether it's with his friends or family (if applicable in the fanfiction's interpretation), are affected. There might also be themes related to the challenges he faces during the pregnancy, like dealing with potential threats in the Underground while being in a vulnerable state. Also, some fanfictions might explore the idea of Sans preparing for the new life that's coming, which could involve everything from gathering resources to making emotional preparations.
A third theme could be acceptance. Maybe the other characters in the Undertale universe have to accept Undyne in this new state. And Undyne herself may have to accept the changes in her body and life. This could lead to some interesting character interactions and growth within the fanfic.
One common theme is probably the exploration of relationships between male characters in the Undertale world. It could be about how they support each other and grow closer.
Another theme is the struggle between good and evil. The ocs can be faced with moral choices, whether to side with the more 'good' characters in Undertale or be tempted by the 'evil' forces. This adds depth to the story and makes it more interesting.
Identity is also a big theme. The undertale characters are put in new situations in the supernatural crossovers, and they start to question who they really are. For instance, if Chara is exposed to a world of angels and demons in a crossover, they might wonder if they are truly a 'bad' character or if they can change and find a new purpose in this new and complex supernatural setting.
One common theme is self - discovery. The monster OCs often start out not really knowing who they are or what they can do, and then through their adventures in the Underground, they learn more about themselves. For example, they might discover hidden powers or a new purpose in life.
The exploration of morality is a theme that can be found. In Undertale, morality is a huge part of the gameplay with choices between mercy and killing. In BATIM, there are also moral dilemmas in a sense, like whether to trust certain characters or not. Fanfics might combine these moral aspects, having characters from both face new moral choices in the crossover world. Also, the theme of discovery is there, as characters discover the new rules and beings of the other's universe.
One common theme is redemption. Many fanfics try to show Chara in a new light, as a character who can be redeemed despite her actions in the game. For example, they might explore her backstory to find reasons for her behavior and then show her growing and changing for the better.
The theme of transformation is also prevalent. It's not just about the physical change from male to female but also the emotional and mental transformation. In many of these fanfics, the character has to adapt to new ways of thinking, new social roles, and new ways of interacting with the world around them. This is often explored through their interactions with the unique and diverse characters in the Undertale setting.
One common theme is female empowerment. These fanfics often show the female monsters being strong, independent, and capable of handling difficult situations on their own. For example, they might be the ones to solve a big problem in the Underground.