The pain of the bones on both sides of the buttocks in the second trimester may be related to the following reasons: 1. The overload of the fetus or amniotic fluid at this time may lead to an overload of the sacroiliac joint on the back side, resulting in inflammation around the sacroiliac joint and causing pain there. 2. It may be related to osteoporosis. The fetus grows and consumes a lot of calcium in the mother’s body, so if the patient cannot keep up with the nutrition, osteoporosis will easily occur, and then bony pain will appear in the lumbar spine, pelvis and lower limbs. 3. 4. The peripheral neuropathy of lumbar disc herniation or sciatica may have occurred in the later stages of pregnancy, which may cause radiating pain in this area and make the patient think that there is bony pain.