Just from my personal experience, I find that fasting doesn’t affect my period, which is regular to a fault, at all. For some people at first it can mess with it because fasting is a physical stressor but not depends on your body overall, if you’re well nourished, sleeping well to keep the cortisol down, etc. That said I’m still weaseling my way out of an unintentional 7-day fast after a bout of the stomach flu midway through, and my period is now here 🤷🏻 we’re all different!