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.
In some fanfictions, Draco hides his pregnancy because he doesn't trust the medical facilities in the wizarding world to handle it properly. He may have heard some horror stories about wizarding pregnancies gone wrong. So, he tries to deal with it on his own, relying on his own knowledge of magic and potions. He might also be hiding it because he's not sure if the father of the baby will be supportive or not, and he wants to first figure out his own feelings and options before facing the father or anyone else.
Chapter 613, "When a Whale Falls, All Life Thrives", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Hearing He Wen's words, Shen Lan was so excited she nearly sprayed...
Chapter 260, "Did You Take All These People Down?!", is the clearest early answer because it turns Chixia Mountain's situation into a concrete plot event. The chapter turns that into something concrete by moving from Yan Dabiao, who deeply felt the sting of deception, flew into a rage...
Chapter 156, "Group Morning Run!", is the clearest early answer because it turns Li Bao's situation into a concrete plot event. Early on, Flash Change, who was always tardy, was late again today.. By the end, Soon after, the meditative He Yun also walked over..
Chapter 96, "Can't Easily Forgive the Scumbag!", is the clearest early answer because Jiang Li gets a payoff that changes the stakes. Early on, Jiang Li's words seemed absurd, yet there also seemed to be a trace.... By the end, Xu Ling couldn't contain her soft sobbing, "How could this happen?.
Chapter 489, "Has She Been Morally Kidnapped?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Yeah," Nina nodded, "I thought you looked pretty imposing, must be.... By the end, However, Nina wasn't willing to give up just yet;
Chapter 283, "You Will Never Understand the Importance of Family!", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. Early on, Shen Lan furrowed her brows as Jiang Shixu suddenly appeared before....
Chapter 265, "This Time You Must Help Me", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with As expected, the truth behind the video was unearthed one by one and...
Chapter 244, "Come, Let Brother Give You a Kiss", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with LOL, Director Sun can't seriously think his sappy quotes are working,...
Chapter 408, "You Can't Be on the Same Team!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Zheng Xun fell silent after he understood the situation.