Congratulations on your pregnancy. Brown discharge is usually caused by a small amount of blood mixed in the normal cervical and vaginal secretions. In pregnant women, a small amount of brown discharge is usually normal. Small blood vessels in the cervix become larger and slightly more exposed as it becomes softer and ready for delivery. These vessels have a tendency to break, release a small amount of blood, and appear as brown discharge. These bleeding
vessels are not a cause of concern.
With that said, all vaginal bleeding needs evaluated by you OBGYN. This is because conditions such as placental previa and placental abruption also cause bleeding but are much more serious conditions that can result in harm or even death to you or your unborn baby. In most cases there are much large amounts of blood (placental previa) and/or some significant pain (placental abruption) associated with each. Your description does not sound consistent with these two.
I suggest that you call your OBGYN's office and report your symptoms. They will give you instructions, and quite possibly ask you to come in for an exam. This way you can be sure that your doctor
knows what is going on with you at each stage in your pregnancy. Good luck.