It is possible that a small biparietal diameter may affect IQ, but it is not absolute, it depends on what caused the small biparietal diameter and how severe it is.
If the smallness of the biparietal diameter of the fetus is caused by fetal developmental delay, this condition may affect the IQ of the fetus. In addition, there is another case of abnormal fetal development, microcephaly, in which the small size of the biparietal diameter of the fetus is more serious, and most of the fetus in this case also has intellectual problems.
Smaller biparietal diameter may also be caused by irregular menstruation, wrong estimation of gestational weeks, and errors in ultrasound measurements, which will not affect the intelligence of the fetus.
During pregnancy, if you find that the biparietal diameter of the fetus is small, you should observe other developmental indicators of the fetus, and if the weight of the fetus is also small, first of all, you need to see if there is an error in the estimation of the week of gestation, and if there is no error, you need to be dealt with under the guidance of the doctor accordingly.