One possible plot could be that after Cinderella marries the prince, she soon discovers she is pregnant. But there are challenges in the palace, like the jealousy of other nobles who might try to undermine her because of her new - found status and the upcoming heir. She has to navigate through court politics while dealing with her pregnancy symptoms.
Well, perhaps in the fanfic, Cinderella gets pregnant unexpectedly. The prince is away on some important diplomatic mission. Cinderella then has to face the scrutiny of the royal court on her own. She might be shunned by some of the more traditional courtiers who question the propriety of her pregnancy so soon. Meanwhile, she tries to keep in touch with the prince through letters, but there are delays and misunderstandings. As her pregnancy progresses, she has to find ways to prove herself worthy and capable of being a future queen, all while carrying the heir to the throne.
In a 'Cinderella pregnant fanfic' plot, it could start with Cinderella and the prince having a very passionate and loving relationship. Then, she realizes she's pregnant. However, there could be a curse or some dark magic at play that endangers her pregnancy. The stepmother and stepsisters might also try to take advantage of her vulnerable state. Cinderella would have to rely on her inner strength, the support of some kind fairy godmother - like figure (maybe a new character introduced in the fanfic), and her love for the prince to protect her unborn child and ensure her place in the kingdom. She might also have to face the possibility of being exiled if the court believes the pregnancy is a bad omen or if her enemies manage to frame her for something.