mucus plug

Question

When a pregnant woman loses her mucus plug, a thick brown stringy discharge accompanied by red blood, does this render her nidda, if she has not yet gone into labor? I have heard that it may not, because it is blood from the cervix rather than the uterus, but this sounds suspicious to me. If it does render her nidda, and she is not in labor yet, when does she do her checks and when can she become pure again?