Brown discharge, normal? No pic

Last week I went in to my doctor's office because of light bleeding/brown discharge. She order an ultrasound to be done and she did a pelvic exam. A nurse called me a few days ago with the results, baby is good, placenta is good, but she mentioned a thicker amniotic membrane. Not sure what that means, I need to go back for an ultrasound at 18 weeks. I'm a little worried about having brown discharge. Any ladies have experience with this?