Under normal conditions, the system will assume that any change in your body weight is related to fat gain or loss. When you select " yes" to the menstrual cycle question at check-in, you’re telling the system that your weight is artificially elevated due to your period. For this reason, if you gain too much weight or don’t lose enough weight as specified by your goal, the system will assume that the cause is fluid gain rather than fat gain and will calculate your new macros accordingly.
Depending on your goal, this may mean that the system holds your macros steady until your body normalizes and your weight change can be assessed without error, or it may mean that it continues to provide the standard weekly adjustment (in the case of reverse dieting). In certain situations, such as when you change your goal, goal setting, or possibly when you edit a check-in, the system may also ask for an "estimated weight." This is simply what you would estimate your weight to be if it weren’t affected by your period, and is used by the system to make more accurate adjustments in the future.