can develop due to invading of placenta into endometrium which can rupture some blood vessels leading to formation of hematoma.
when placentas lying near to cervical os in around 3cms then it can be taken as placenta previa
but usually with progression of gestational age
the placenta can migrate,
it can sometimes cause painless profuse bleeding and sometimes spotting,
so your brown discharge could be spotting mixed with mucus or just because of increased blood supply to local organs.
anyhow until there is no bleeding and baby condition is good no need to worry, but be cautious if bleeding occurs immediately contact doctor
and avoid journey, intercourse, weight lifting etc which can cause bleeding.