18 weeks pregnant stomach a little hidden pain, black stool may be related to diet and other physiological reasons, may also be caused by gastrointestinal bleeding and other pathological reasons, need to be analyzed. 1. Physiological reasons: pregnant women consume a lot of iron-rich foods or drugs, will add a certain burden to the gastrointestinal function, resulting in pregnant women with a vague pain in the stomach, and also black stools. 2. Pathological reasons: If a pregnant woman has chronic gastrointestinal bleeding, or gastric ulcer and other diseases, there will also be a vague pain in the stomach and black stools. If you are pregnant for 18 weeks and have a vague pain in your stomach and black stools, you should go to the hospital for a timely examination to clarify the cause of the disease and treat the symptoms under the guidance of the doctor.