If you are 18 weeks pregnant and have a paroxysmal hardening of the stomach, it is not normal. It is best to go to the hospital for an obstetrical ultrasound and to hold your urine when checking the ultrasound so that the ultrasonographer can look at the length of the cervical canal and whether the opening of the inner cervical opening has opened up. If the cervical canal is shortened and the opening of the uterus is enlarged, and there is a hardening of the belly, it may be a preterm miscarriage. However, generally speaking, fetal movement starts to appear around 18 weeks of pregnancy, and sometimes fetal movement bulges one of the limbs or parts of the fetus and manifests as hardness on the surface of the belly. This may be caused by the fetal movement, so it is safest to go to the hospital to check the ultrasound, as long as the ultrasound indicates normal, no special treatment is needed, regular prenatal checkups can be done.