Well, sometimes in these fanfictions, Draco hides his pregnancy because he's in a secret relationship. For example, if he's in love with someone from a different house or a different social standing. He might be afraid that if his pregnancy is discovered, the relationship will be exposed and there will be consequences. Also, there could be a plot where he discovers some magical mystery related to his pregnancy that makes him think he has to hide it, like there's a prophecy or some dark magic involved.
Draco and Hermione is also a pairing that shows up in these types of fanfictions. Given their different backgrounds and the initial animosity between them in the original story, Draco hiding his pregnancy from Hermione can lead to a complex story. It could involve Hermione slowly finding out and having to deal with her own feelings about Draco and the situation. There's also the pairing of Draco with a random Muggle, which can add a whole new layer as he has to hide it not only from the wizarding world but also from the Muggle's family and society.
One possible reason could be fear. Maybe Hermione is afraid of how Draco will react. Given their history in the wizarding world, with their houses being rivals at Hogwarts and different beliefs, she might think he'll reject her or the baby. Another reason could be to protect the baby. There could be some threats or dangerous situations in the wizarding world that she believes Draco's knowledge of the pregnancy would put the baby at risk. For example, there could be dark forces that target the children of powerful witches and wizards, and she wants to keep the baby's existence a secret to safeguard it.
She could be using magic. For example, she might use a concealment charm on herself to hide any physical signs of pregnancy. Hermione is very skilled in magic, so she could easily cast spells that make her look the same as before. She might also avoid situations where she would be in close contact with Draco. By staying away from him as much as possible, she reduces the chances of him noticing any changes in her.
Well, in fanfiction, Draco's character can be quite interesting when paired with the theme of pregnancy. It could be a story of him being supportive or struggling with the situation. It gives the writer a chance to show different sides of Draco.
A Draco - Ginny pregnancy fanfiction could involve a lot of drama. For example, Ginny might struggle with how the wizarding community would view her being pregnant with Draco's child. And Draco might have to face his own prejudices and those of his family. There could be a lot of tension and character development as they navigate this new situation together.
Maybe Sakura is afraid of the reaction from her friends or family. So she decides to keep her pregnancy a secret at first. She might be worried about how people will judge her, especially if she's in a difficult situation like not being in a stable relationship.
Perhaps he has hidden the full extent of his Sage Mode abilities. He might be able to stay in Sage Mode for a much longer time than he has shown so far, or have some other undisclosed powers within that mode, like the ability to sense enemies from much greater distances.
Her closest friend might help her. Someone who really understands her and is willing to keep her secret. For example, Ino could be that friend. Ino has always been there for Sakura in the original story, so she could be the one to support Sakura during this difficult time.
Yes, there are likely fanfictions out there about Draco Malfoy and Ginny Weasley and the concept of pregnancy. The Harry Potter fandom is quite large and diverse, with fans exploring all sorts of alternative storylines. However, not all fanfictions are easy to find. You might want to start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own (AO3).