One common theme could be the conflict between their two very different worlds. Batman's sense of justice and the Joker's chaos, and how a pregnancy would further complicate that. Another theme might be the emotional turmoil for both characters, as it's a very unexpected situation for them.
There might not be a large number of such specific 'fem joker x batman pregnancy fanfic' readily available. However, you can start by looking through Batman - related fanfic communities. Some writers might be exploring unique pairings like this, but it could take some time to find exactly what you're looking for. You may also want to set up alerts on these platforms so that if a new story of this type is posted, you'll be notified.
Another plot could involve Fem Batman and Joker in a sort of cat - and - mouse game where the Joker is specifically targeting Fem Batman because she's a woman. He might underestimate her at first, but she proves to be just as formidable as the male version. This would explore themes of gender and power in the Batman - Joker dynamic.
In the original, Batman and Joker have a very adversarial relationship based on male - male dynamics. In the fem version, Fem Batman might be more empathetic towards the Joker, which could either make the relationship more complex or lead to different outcomes in their battles.
At first, Batman would be shocked and a bit confused. But quickly, his detective instincts would kick in. He would view the deaged Joker as a new and unexpected threat. He'd start gathering information about how the deaging occurred, in case it was part of a larger, more sinister plan. Batman would also be very careful not to underestimate the deaged Joker. He might try to isolate the Joker in a secure location to prevent him from causing chaos in Gotham. And he'd be constantly on the lookout for any signs of the Joker's old, devious self emerging from this new, younger form.
In the traditional Batman comics, the relationship between Batman and the Joker is not a love story. It's more of a complex adversarial relationship. The Joker is a villain who constantly tries to cause chaos and harm, and Batman is dedicated to stopping him.