Lower tummy pain during the 18th week of pregnancy can be a common symptom, but it is important to pay attention to any unusual or severe discomfort. In most cases, this type of pain is caused by the expanding uterus putting pressure on the ligaments and muscles in the lower abdomen. This can be particularly noticeable when you have a full bladder, as it adds additional pressure. However, if you experience severe or persistent pain, accompanied by other symptoms such as bleeding or fever, it is important to consult your healthcare provider. They will be able to evaluate your specific situation and provide appropriate guidance. It's always better to err on the side of caution and seek medical advice if you are unsure about any symptoms you are experiencing during pregnancy.