It is completely normal for your stomach to become hard during pregnancy, especially around the 18-week mark. This sensation is often referred to as "Braxton Hicks contractions" and is your body's way of preparing for labor. These contractions are usually painless and irregular, but if they become frequent or painful, it is important to consult with your healthcare provider. Staying hydrated, changing positions, and practicing relaxation techniques can help alleviate discomfort caused by these contractions. Remember to always reach out to your healthcare provider if you have any concerns or questions about your pregnancy.