The head circumference of a fetus at 21 weeks is usually around 18cm which is normal, but it varies from fetus to fetus depending on the nutritional supply of the mother and its own development. Fetal head circumference is directly related to fetal growth and development. A large head circumference indicates a large gestational period and rapid growth and development. Usually, the head circumference of a 21-week-old fetus is about 18 cm. However, due to the different nutritional supply of the mother to the fetus and the different developmental level of the fetus, the head circumference of a 21-week-old fetus may be around 18 cm, with a range of 1~2 cm. If the head circumference of the fetus is too large or too small is an abnormal sign, and the cause should be actively searched for. A large fetal head circumference may be caused by excessive fetal growth, while a small head circumference may be due to factors such as poor brain development. In either case, it is important to actively seek medical attention to identify the cause and then target treatment.