Biparietal diameter small head circumference of 4 weeks, whether it is normal needs to be further analyzed, may be caused by fetal developmental delay or congenital brain dysplasia. 1. Fetal developmental delay: ultrasound examination found that the fetal biparietal diameter small head circumference of 4 weeks, this situation if the results of the Down’s syndrome screening is normal, at the same time other obstetric results are also normal, this situation is generally considered to belong to the fetal developmental delay induced, it is recommended that the pregnant woman does not need to be overly worried about it, can be supplemented with nutrients, continue to receive observation. 2. Microcephaly: If the result of Down’s syndrome screening is abnormal, it is generally considered to be microcephaly, which may be related to congenital brain dysplasia and toxoplasmosis infection, etc. It is recommended to undergo further examination. In addition, if the fetus is diagnosed with microcephaly, it is advisable to consult a specialist and, if necessary, terminate the pregnancy at an early stage.