The biparietal diameter is the distance between the parietal bones of the fetus. The size of the biparietal diameter is usually used to determine the growth and development of the baby and whether the pregnancy is normal. If the biparietal diameter is large, there may be several reasons, one is the error in the calculation of the gestational week, because some mothers may have irregular periods, or wait until a long time after the menstrual period and find that it has not come before projecting, the pregnancy time may be late, so the calculated gestational week is inaccurate and may be smaller than the normal gestational week. The baby’s biparietal diameter may be larger compared to normal gestational age. If the mother consumes too much nutrition during the pregnancy, the fetus may have a large biparietal diameter and the diet needs to be adjusted to keep the fetal development within the normal range. The other condition is fetal malformation, if the fetus has abnormal brain development, it will also appear to have a large biparietal diameter.