The hardness of the stomach in five and a half months of pregnancy may be contractions, which may be caused by recent overwork, keeping the same position for a long time or sudden movement. 1. Overwork: The fetus grows faster in mid-pregnancy, and the pregnant woman has a heavy burden. If a pregnant woman is overworked for a period of time, it may lead to contractions. Pregnant women can prevent contractions by getting enough sleep and avoiding exertion. 2. Keeping the same position for a long time: If a pregnant woman keeps the same position for a long time, such as lying on her side for a long time, and does not pay attention to change the position at the right time, it may trigger contractions. 3. Sudden movement: If a pregnant woman suddenly and vigorously exercise, such as running, coitus, etc., can lead to uterine stimulation, contractions. Five and a half months pregnant contractions may also be caused by other reasons, if the frequency is too high, and the pain is obvious, it may also be other reasons, it is recommended to go to the hospital for examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment.