The pain in the bones of the buttocks in the fourth month of pregnancy may be related to the following reasons: 1. It may be caused by prolonged sitting and standing. In this month, pregnant women are often used to sitting and standing for a long time, which will lead to repeated friction and squeezing of the skin here, resulting in skin damage and even inflammation around the sciatic nodes, which will lead to pain around the sciatic nodes. 2. This condition may be caused by osteoporosis. The rapid growth of the fetus often leads to insufficient calcium supply in the body, which in turn leads to osteoporosis, which often produces bony pain in the pelvis, hip joint and lower limbs. 3. This condition may be caused by inflammation of the joints related to the lower limbs, such as the rapid increase in the fetus and weight, which may lead to excessive weight bearing on the lower limbs, which may cause inflammation of the hip joint, sacroiliac 4. This condition may also be caused by lesions related to the tailbone, for example, such pregnant women may develop anal lesions such as hemorrhoids, which may then stimulate the tailbone, resulting in pain around the tailbone.