The placenta is abnormally positioned in the uterus near the cervix. Only when the placenta is so close to the cervix that it can get pulled away from the uterine wall can it cause problems. As the cervix starts to dilate in the third trimester, the placenta becomes disrupted and separates from the uterine wall, resulting in vaginal bleeding. If you experience any vaginal bleeding, report this to your doctor immediately. Sometimes a low lying placenta can be seen on the ultrasound and not cause any problems or move slightly farther up the uterus during pregnancy so it is not as low lying.