It is not uncommon for pregnant women to experience light bleeding during the third trimester, including at 31 weeks. While it can be alarming, there are several potential causes for this symptom that are generally harmless. These include cervical irritation, hormonal changes, or even sexual intercourse. However, it is always important to consult with your healthcare provider to rule out any serious complications. They will be able to evaluate your specific situation and provide appropriate guidance and care. Remember, it's better to err on the side of caution when it comes to your health and the well-being of your baby.