What is Biparietal Diameter? Fetal biparietal diameter is the distance between the parietal bullae, which is actually the transverse diameter of the fetal head. The size of the biparietal diameter is measured by ultrasound, the method is to measure the distance between the two parietal bulges on a standardized plane, usually choose the thalamus plane, the measurement is the extension of the skull, usually more often used to measure the proximal side of the outer edge of the skull to the distal side of the skull between the inner edge of the measurement of the distance. Scalp tissue is not included, as there are thick and thin scalp tissues that create gaps. The appropriate size at full term is approximately 9.3 centimeters. Note: The value of the measurement is related to the shape of the head, whether the baby is in the pelvis and the position of the fetus, as well as the experience of the ultrasound doctor, so in order to minimize the error, you can take three measurements and take the average value. What is the standardized comparison table of fetal biparietal diameter? The fetal biparietal diameter chart mainly shows the size of the biparietal diameter at different gestational weeks. In fact, there are several criteria for evaluating fetal growth and development in more detail: one is the standard deviation of the mean value of the biparietal diameter. If the mean value is greater than two standard deviations, it indicates a larger diameter, and if it is less than two standard deviations, it indicates a smaller diameter. There is also an evaluation of what percentile of the normal value the biparietal diameter value is in. The overall size of the head is mainly determined by the head circumference, not the biparietal diameter. A large head, i.e., a relatively large fetal head, is defined as a head circumference value in the 97th percentile or above, while a small head is defined as a head circumference value in the 3rd percentile or below. Before 13 weeks, the size of the fetus is mainly determined by the parietal diameter, and the biparietal diameter is measured after 13 weeks. Generally speaking, the biparietal diameter is about 23mm at 13 weeks, 47mm at 20 weeks, 73mm at 28 weeks, 91mm at 36 weeks, and 93mm at full term. Attention: the size of the biparietal diameter should be evaluated by firstly checking the week of pregnancy. If the ultrasound result is larger than the actual week of pregnancy, it may need to be noted that If the ultrasound result is larger than the actual pregnancy week, you may need to pay attention to whether the biparietal diameter is too large, but also pay attention to the head circumference, generally 12 to 28 weeks of the biparietal diameter measurements are closest to the pregnancy week, and the significance of the guidance of the pregnancy week is also relatively strong, to the late pregnancy, the biparietal diameter measurements will be biased, the evaluation of the size of the fetal head will not be as accurate as the indicators of head circumference.