A biparietal diameter of 21mm at 13 weeks of conception is considered normal. As long as the biparietal diameter of the fetus at 13 weeks of conception is between 15 and 25mm, then it is considered normal development. Because different fetuses grow at different rates, even if two women are at the same gestational week, the biparietal diameter may vary, as long as it is within the normal range. Biparietal diameter is the widest distance between the two sides of the head of the fetus, reflecting the size of the head of the fetus, you can judge the development of the fetus according to this indicator is consistent with their own substantial gestational weeks. It is important to take regular pregnancy checkups during pregnancy to determine the growth and development of the fetus, and to identify the cause of any abnormality as early as possible and intervene as soon as possible to avoid delaying the condition.