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.
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.
One possible reason for Sakura to hide her true strength in fanfiction could be for self - protection. Maybe she is in a world where showing too much power makes one a target. For example, if she is in a political intrigue - filled setting, she might keep her strength under wraps to avoid being used by various factions.
Hermione could be hiding her pregnancy from Snape because she's not sure of his feelings towards her. There may be an ambiguous relationship between them. She might be in love with him, but not sure if he feels the same. And she doesn't want to burden him or make things awkward between them. Or perhaps she suspects that Snape might be involved in something dangerous, and she doesn't want her unborn child to be dragged into it.
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.
Perhaps Sakura is dealing with family issues. She could be hiding the pain of having a difficult family situation, like an overbearing parent or a family member with an illness. In the fanfiction, she might focus on her school or ninja work to distract herself from her family problems.
One character could be Naruto. He might be Sakura's friend, but completely oblivious to her hidden pain. He's so focused on his own goals that he doesn't notice Sakura's struggle, which could add to her feelings of loneliness.
A common plot device is the 'big reveal'. Sakura's true strength is suddenly shown in a moment of great need, like when her friends are in extreme danger. This creates a dramatic effect and often turns the tide of the story.
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.
Maybe she wants to avoid causing unnecessary worry among her friends and comrades. They are often involved in dangerous missions, and if they knew she was pregnant, they might get distracted. Naruto might think it's better to keep it a secret until she figures out how to handle the situation without disrupting their important work.