You may stutter when stressed as stress has a negative response on your body which may cause anxiety and tension. The tension caused by anxiety can appear in the chest, shoulders, neck, jaw, tongue and/or lips. As stress can cause tension in the body it can also aggravate the stutter that is already there. When someone experiences tension due to stress their muscle movements controlled by the brain moving the areas for speech such as tongue, lips, neck and jaw can become more tense and result in a stutter.