Translation:Shulchan Aruch/Yoreh Deah/183

1. A woman who has a discharge of blood from her uterus, whether due to outside factors or purposefully, is tameh (impure) if she felt it flowing out. If she felt the discharge she is tameh even if the blood did not flow out of her body, and even if she only saw a drop of blood the size of a mustard seed; she must observe seven clean days for it. RAMA: as will be explained in Siman 196. And there is no distinction between a married and an unmarried woman with regards to the laws of niddah (Rivash in Siman 425 as brought by Bet Yosef), because all who have intercourse with a niddah are liable for karet.