Normal head circumference and small biparietal diameter indicate that fetal development may be normal. Clinical measurement of fetal head circumference and biparietal diameter is commonly used to determine whether the fetal size is consistent with the gestational week. Head circumference is the length of the head from the occipital node to the upper edge of the arch of the eyebrow, and the biparietal diameter is the widest distance between the right and left parietal bones of the fetal head. By measuring the size of the head circumference and biparietal diameter, we can roughly estimate the week of gestation of the fetus, and thus determine whether the size of the fetus is consistent with the week of gestation. However, due to individual differences, some fetuses have narrower left and right diameters and longer anterior and posterior diameters, which results in normal head circumference and small biparietal diameter. It is recommended that pregnant women undergo timely screening tests during pregnancy to rule out fetal developmental abnormalities. If the test result is normal head circumference and small biparietal diameter, and there is doubt about the result, you can go to a large hospital for further examination to rule out fetal development with or without malformation, if the test result is normal, there is no need to worry.