If a woman thinks about sex and has discharge, either because of the thoughts or because of her natural menstrual cycle, is it necessary for her to do ghusl? Furthermore, if she is fasting, is her fast invalidated?
It is najis if the discharge is the result of an orgasm (i.e. sexual arousal), and therefore requires ghusl, and also invalidates fasting. If the discharge is not due to sexual arousal but it came out in large amounts, based on obligatory precaution it is deemed najis and requires ghusl, and invalidates fasting. If it was not due to sexual arousal and the amount of discharge is very minute it is considered tahir and does not require anything.