Well, it's difficult to say whether there is such a fanfic. Fanfic writers come up with all sorts of creative ideas, but this particular combination of Martian Manhunter and M Gahn in a pregnancy - related story might not be very common. There are so many different fanfic universes and themes out there. It could be that someone has written it, but it might also be that no one has thought of it yet or it's just not popular enough to be easily found.
One possibility could be a story exploring the challenges and joys of impending parenthood in a unique context. Since Martian Manhunter has his own set of powers and alien biology, it could focus on how they deal with the pregnancy considering these factors. Maybe they have to find a way to ensure the baby is healthy given their different species' characteristics.
It could be about how they deal with the unexpected pregnancy. Maybe they face challenges from their respective communities or families. Or it could explore the unique biology of Martian Manhunter in relation to the pregnancy if he is the one carrying the child.
Their relationship could grow stronger as they face the pregnancy together. They might become more reliant on each other, sharing fears and hopes about the future. For example, Martian Manhunter could use his telepathic abilities to connect with the unborn child, which could bring them closer as a couple.
One possible plot is that Martian Manhunter discovers Meghan's pregnancy has some sort of alien connection. He then has to figure out how to deal with this while also keeping her and the unborn baby from being used in some sort of intergalactic experiment. He might enlist the help of other superheroes he knows to ensure their safety.
It could be some alien species that are jealous of the special child Meghan is carrying. They might see it as a threat to their own existence or power in the universe.
One possible scenario in this fanfiction could be that Martian Manhunter discovers Alex's pregnancy and uses his unique Martian abilities to protect her. Maybe he can sense any potential threats to her and the unborn child before they even occur.
One possible plot could be that they face challenges in protecting their unborn child from enemies who might want to use the baby's powers for evil. Since both are Martians, their child would likely have unique and powerful abilities that could attract unwanted attention.
Their superhero friends would probably be very supportive. They would offer help in various ways, like setting up a secure place for the baby or providing protection. For example, Superman might offer his Fortress of Solitude as a potential safe haven if needed.
Well, they might go on adventures while Alex is pregnant. Martian Manhunter could use his powers to make sure Alex stays safe during their travels. For example, he could fly her to a safe place if there's any danger.