Charting tip: On your “Light” and “Very Light” days of true menstruation*, you can check for and chart cervical mucus the same as any other time of your cycle. During days of “Heavy” or “Moderate” flow, you won’t be able to tell whether cervical mucus is being discharged.
But on those days when your menstrual flow is light enough that you CAN check for cervical mucus, DO!
Likely you will have a “B” for brown or “R” for red, since you’ll see the color when you wipe, but as long as you don’t see cervical mucus, these are infertile days.
(*If you don't know what a "true menstruation" is, make sure you find out, before you consider any days of bleeding infertile.)