Fetal biparietal diameter head circumference is small may be pregnancy week accounting error, genetic factors, malnutrition, developmental malformations and other reasons. 1. Wrong accounting of gestational weeks: women’s usual menstrual cycle disorders, menstrual cycle time is long, ovulation time is relatively backward, after coitus fertilized egg implantation time will be delayed, there will be the actual gestational weeks of the fetus than the last menstrual period of the projected gestational weeks of the small, fetal biparietal head circumference of the phenomenon of the small. 2. Genetic factors: if there is a genetic factor in the family that makes the head small, the fetus is also prone to small biparietal diameter. 3. Malnutrition: excessive picky eating or dieting, poor gastrointestinal function during pregnancy may lead to maternal malnutrition, which indirectly leads to low absorption of nutrients in the fetus, and slow growth of the fetus, which may result in small biparietal head circumference. 4. Developmental malformations: chromosomal abnormalities in the embryo, viral infections during pregnancy and other reasons may lead to fetal microcephaly, manifesting the fetal biparietal head circumference is small. Fetal biparietal head circumference is small due to a variety of reasons, serious cases are recommended to consult a doctor in time for examination and treatment.