Fred's reaction could be a complex mix of emotions. On one hand, he might be overjoyed at the thought of having a child with Hermione. He may immediately start making plans for their future as a family. However, he could also be a bit worried. After all, having a baby is a big responsibility. But his natural optimism would likely shine through, and he'd assure Hermione that they can handle it together. He might also crack a few jokes to lighten the mood, saying something like 'Our baby will be the most mischievous one in Hogwarts before we know it!'
Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
Well, Hermione's usually logical and rational, but the pregnancy can make her more emotional at times. This is a new side of her that the fanfiction can explore. Fred, on the other hand, may find himself having to grow up quickly. He has to think about providing for his family now, which is a big change from his carefree days at Hogwarts.
Fred often uses his humor to diffuse tense situations that Hermione might be in. For example, if she's being harassed by some bullies, he'll make a joke that makes the bullies look stupid and also makes Hermione feel better.
In some fanfics, Harry might be able to tell Fred and George apart by their different interests within the story. For example, Fred could be more interested in potion - making experiments that are a bit on the wild side, while George is more into the business side of their joke shop plans. So when they talk about their hobbies or future plans, Harry can distinguish between them. Also, their sense of humor might be slightly different. Fred might crack a certain type of joke more often, like those related to magical creatures in a really absurd way, while George prefers jokes about Hogwarts professors. Through these nuances in their personalities, Harry is able to tell them apart.