Quick! Name the last book that you read that the MC went to the bathroom. Can’t think of one? I doubt you are alone.
For me the book that immediately comes to mind are Tamora Pierce books. Not all of them include this little detail, but most do. The female protagonists even have periods!
It might seem weird, but just the off-handed comment about the main character going to the bathroom or needing to pack extra clothes right before a last minute trip because they are expecting their period makes me super happy. It makes the protagonists real people! It doesn’t demonize the fact that as a woman I have a period and have to think about the consequences of that. It normalizes menstruation for young girls who are self conscious of it.
The fact that most authors ignore this very common, realistic, mandatory body function frustrates me. Your main character eats, otherwise readers would find it weird that they didn’t so why can’t they also go to the toilet? Sure there is a bit of a taboo about this, but including it, as I mentioned before, normalizes it and actually helps young people with their body image.
Don’t know how to include it in your story?
How often have you had an interesting conversation in the washroom? If you are in high school, a setting in many contemporary books, I would hazard it would be fairly often because you have some limited privacy there. When you are about to go into battle battle or slay a dragon how likely is it to have a nervous bladder? A long run from a monster poses a challenge for woman that men don’t tend to have. Ditto long treks or hikes or journeys with a mixed group of woman and men.
Just think of all the awkward situations you can get your characters into! Think of the plotting potential!