I'm working on a fantasy novel set in a place with a rough estimate of 17th century on a time period and technology scale. My main characters are two women, and during their adventures (which cover the course of nearly two months) they each experience their menstrual cycles.

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One of the women is royalty. She was raised pampered, she was well fed, healthy, until the time in the story, where she lacks the resources and comforts she is used to. I imagine (from the perspective of a trans woman, I've never had a period) that she has somewhat typical periods as far as cramps and pain.

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The other woman is a few years younger, and regularly walks the line bordering malnourishment. She is underweight, and often underfed, coming from a very different background (part of the story is the differences between the women and their upbringing).

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I don't remember where I read this, heard this, or whatever, but I have this idea in my head that underweight and poorly fed women have lighter and less consistent cycles with varying blood flow. Is this something I imagined I heard somewhere, or is there truth to it? Any help on the matter would be lovely, though this is a small detail in the story, I would like to get it right. Thanks in advance.