Yes, there are many Sherlock Molly pregnant fanfictions. Writers like to create this scenario to see how the characters, especially Sherlock, will react to such a life - changing event. It can also be a way to explore Molly's character more deeply, as she navigates motherhood and her relationship with Sherlock.
Well, a frequent theme is the exploration of Sherlock's emotions. Usually, he's depicted as not very in touch with his feelings, but a pregnant Molly can change that. He might start to feel protective, loving, or even scared. Also, there's often the theme of family, whether it's Sherlock coming to terms with the idea of having a family or Molly's own family's reaction.
Sherlock might also feel a sense of responsibility, even if the pregnancy wasn't his doing. Given his loyalty to his friends, he would probably try to be there for Molly. He could use his connections to ensure she has the best medical care available. And in the long run, this could be a turning point in his relationship with Molly, either bringing them closer together or causing some strain depending on how he handles the situation.
Writers often develop their relationship by showing Sherlock's initial confusion and struggle with the new situation. He might be out of his depth when it comes to dealing with Molly's pregnancy, which leads to some interesting interactions. They also might show Molly being patient with Sherlock as he tries to come to terms with becoming a father. For example, she could gently guide him through the process of doctor visits and preparing for the baby.
Their relationship could become more intimate. Sherlock, who is often aloof, might become more affectionate towards Molly. He would start to worry about her well - being and that of the unborn child, which could lead to him sharing his feelings more openly.
One possibility is that it could lead to a significant change in Molly's character. She has always been in Sherlock's shadow to some extent. Now, being pregnant, she would have to become more assertive. She might decide to step away from the dangerous cases Sherlock gets involved in to protect her unborn child. This could also cause some friction between her and Sherlock if he doesn't understand her decision.
John would probably be shocked at first. He'd be concerned about how Sherlock would handle being a father, given his rather detached nature. But then he'd be very supportive, offering practical advice like setting up a nursery. Mrs. Hudson would be over the moon. She'd start knitting baby clothes right away and would constantly be fussing over Molly, making sure she's eating well and resting enough.
One interesting plot twist could be that Sherlock initially doesn't believe he's the father and tries to use his deductive skills to disprove it, only to find out he was wrong all along. Another could be that Molly tries to keep the pregnancy a secret from Sherlock at first, but then circumstances force her to reveal it in a very dramatic way, like during a dangerous case. And sometimes, there might be a plot twist where other characters, like Mycroft, try to interfere with the situation for their own reasons, either to protect Sherlock or because of some secret agenda they have regarding the potential child.
Perhaps in the fanfiction, the pregnancy is a result of some kind of experiment gone wrong. Sherlock then has to use his brilliant mind to figure out how to ensure the health of both Molly and the baby. He might have to face his own fears about becoming a father and how it will change his life and relationship with Molly.
In Sherlock fanfiction, the idea of Sherlock as a family man often involves a shift in his character. If Molly is part of this scenario, it could be a really interesting dynamic. Maybe Sherlock's deductive skills are used to understand family matters in a new way, with Molly being a grounding influence. For example, in some fanfics, they might have a child together and Sherlock has to learn to be more empathetic and less cold, with Molly guiding him through the process.