The word "panties" is typically used in the plural, although it is singular in meaning, meaning one pair of women's lower undergarments. " The word "panties" - and "underwear" too - is often used in a ribald sense, and so it would probably not be appropriate for a formal article such as you describe. In such an article you would use technical/scientific-type language such as: undergarments stained as the result of menstruation.
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