If the fetal B-polar diameter value is less than normal, the fetus is said to be small, mostly due to fetal microsomia. However, severe biparietal hypoplasia may be combined with microcephaly, but there is no specific gestational week limit. First of all, some pregnant women have late conception or long menstrual cycle, so the calculated gestational week may be larger than the actual gestational week, and then the small biparietal diameter is caused by the error of gestational week calculation, which should be corrected by verifying the gestational week. The size of the fetal biparietal diameter varies from 9.3 cm at full term to 7 cm at 28 weeks of gestation and 8 cm at 32 weeks of gestation, and the small size of the fetus is not only determined by the biparietal diameter but also by the estimated fetal weight on ultrasound. If the ultrasound predicts that the fetal weight is less than 10% of the normal weight at the corresponding gestational week, the diagnosis of fetal growth restriction is made. In addition, severe biparietal hypoplasia may be combined with microcephaly, but there is no clear evidence as to how much biparietal hypoplasia may be combined with malformation. In conclusion, the presence of microcephaly depends on the clinical manifestations and the screening tests during pregnancy. Therefore, if the fetal biparietal diameter is really small, pregnant women should pay attention to more rest and increase nutrition appropriately. If fetal malformation is suspected, karyotype screening and 4D ultrasound should be done after 12 weeks of pregnancy.