A baby’s small head may be caused by genetic factors, chromosomal mutations, abnormal bone development and so on. 1. Genetic factors: In most cases, the baby’s head is small, if there is no other uncomfortable symptoms and heredity has a great relationship, may be because the father or mother have children with small head size. 2. Chromosomal mutation: It may be due to the chromosomal mutation that induces microcephaly when the pregnant woman is affected by virus, bacteria or radiation during pregnancy. 3. Abnormal bone development: It may be due to abnormal bone development after birth, resulting in premature closure of the skull area caused by the phenomenon of small head. There may be other reasons for the baby’s small head, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.