Hello and welcome to HCM
Thank you for your query
Firstly I would like you to tell that,there may be chances of
pregnancy,as brown
vaginal discharge is seen in
implantation bleeding,being an early sign of pregnancy.
To rule this out,I would recommend you to go for Pregnancy test.
Secondly,if its not pregnancy,then It can be nothing but old endometrial tissue.In women, the discharge is the body's way of releasing any old endometrial tissue that was not shed during the menstrual cycle. its common to get light brown discharge before menstrual cycle.
Usually Most light brown vaginal discharges are normal,and go off within few days
but if they its extremely dark brown,then its a sign for some illness.
for eg. endometrial(lining of the uterus) bleeding
pelvic inflammatory disease.
or at the worst
cervical cancer.
If the discharge still persists,You need to check with your gynecologist.
Thank You.
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Good luck And Stay healthy.