My period was late and I thought I was pregnant, but 10 days later I passed some blood that was brown in colour, and on another day it was in the form of a black thread. What should I do?.
The blood which means that a woman cannot pray or fast or have intercourse is menstrual blood. Allah has given it signs which are well known to women. With regard to colour, it is black or dark red; with regard to smell, it has a unpleasant odour, and it is also thick. It is blood that comes from the uterus so it cannot be one or two drops and it cannot be like a thread.
You are saying that you passed blood that was brown in colour; if you mean that it was brownish, if it comes before the period it is not regarded as menses. If you meant that the colour of the blood was light, then if this was a flow of blood and not just a trace or a few drops, then the basic principle is that it is menses.
As for the black thread, it is not regarded as menses.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: If a woman passes a few drops of blood during the day in Ramadan, and this continues with her throughout the month of Ramadan and she is fasting, is her fast valid?
He replied: Yes, her fast is valid. These drops of blood do not matter because they are from a vein. It was narrated from ‘Ali ibn Abi Taalib (may Allah be pleased with him) that he said: These drops which are like a nosebleed are not menses. This is what was narrated from him, may Allah be pleased with him. End quote from 60 Su’aalan fi’l-Hayd, p. 6
And Allah knows best.