A novel is not a travel guide or ethnological treatise that describes a place and the everyday life of its people in all its aspects. A novel is a narration of everything that is relevant to its story. The narrative does not contain anything that is self-evident, obvious, banal, and commonplace for the reader such as that people have hair or need to sleep or that women menstruate. Only when there is a deviation from the norm, such as that a person loses their hair or a character cannot sleep or menstruation is handled differently in the fictional world – that is, whenever something normal and common becomes abnormal and uncommon – or if it is relevant to the story – that is, whenever something normal and common affects the progression of the plot – are they mentioned in the narrative.
This doesn't, of course, mean that you mustn't write a story in which the period of a character is mentioned, even if it is commonplace.
- Just like a detective's alcohol problem or a sword fighter's love affair can either serve to develop the character more fully or become an obstacle for their goals, the sword fighter's menstrual cramps might give their enemy an opportunity to escape or show a part of the detective that makes her more real to the readers.
- And menstruation or defecation might be a theme for a narrative.
When I was dating a single mother and getting involved with her baby daughter as an ersatz father of sorts in the late 1980s, most fantasy still was about untethered heros unimpeded by the demands of family life. From my personal experience, I began a fantasy novel about a wandering single mother sword fighter. Fantasy literature has vastly changed since then and there are many fantasy novels about mothers, fathers, and what having a child in a fantasy world means by now, but menstruation and defecation are still a rare topic, so if you have something interesting to say about these topics, that might result in a fascinating read.
But be careful not to make it boring or gross. Most people don't want to read about feces. And don't put something in a story just because it is a normal part of life and most stories don't mention it. A detective who constantly brushes their teeth, however medically sensible in real life, will quickly begin to irritate your readers. Think about how fantasy writers include the saddling of horses or getting dressed or preparing meals into their stories: sometimes these menial tasks are mentioned, usually while the characters talk, and usually in passing. Handle menstruation and defecation the same way and you will be on the safe side.