The causes of small fetal head in late pregnancy are: 1. malnutrition during pregnancy, gestational diabetes, fetal malformation, etc. If the pregnant woman does not rest, the fetus will be malnourished in the womb, so the fetus will have a small head in the second trimester. 2. If the pregnant woman has gestational diabetes, the fetus will grow rapidly to form a huge child, and the fetal head will grow relatively slowly, so the fetal head will be small. 3. The fetal head will also appear small in the second trimester. If the fetal head is small in late pregnancy, it is normal within one week and there is no need to worry too much. If the fetal head is small for more than one week, it is recommended to have a relevant examination and if it is caused by diabetes, it is recommended to use insulin to control the blood sugar within the normal range.