Biparietal diameter is the distance between the two parietal bulges, which is the largest transverse diameter of the fetal head. When the pregnancy is more than 14 weeks, the measurement of biparietal diameter can be used together with other indicators such as head circumference to comprehensively determine the gestational age. Different weeks of pregnancy correspond to different biparietal diameters, as follows. 1. Fetal biparietal diameter at 14 weeks of gestation is 2.1-3.2cm, at 16 weeks of gestation the range of biparietal diameter is 2.7-3.9cm, at 18 weeks of gestation the range of biparietal diameter is 3.4-4.5cm, at 20 weeks of gestation the range of biparietal diameter is 4.1-5.2cm, at 22 weeks of gestation the range of biparietal diameter is 4.7-5.9cm, at 24 weeks of gestation the range of biparietal diameter is 5.3-6.5cm, at 26 weeks of gestation the range of biparietal diameter is 5.9-7.2cm. 7.2cm. 2. 28 weeks of gestation, the range of biparietal diameter is 6.5-7.7cm, 30 weeks of gestation, the range of biparietal diameter is 7.0-8.3cm, 32 weeks of gestation, the range of biparietal diameter is 7.5-8.8cm, 34 weeks of gestation, the range of biparietal diameter is 7.9-9.2cm, and 36 weeks of gestation, the range of biparietal diameter is 8.2-9.6cm. 3. 37 weeks gestation biparietal diameter range 8.3-9.7cm, 38 weeks gestation biparietal diameter range 8.5-9.9cm, 39 weeks gestation biparietal diameter range 8.6-10cm, 40 weeks gestation biparietal diameter range 8.6-10.1cm. Biparietal diameter fluctuates during pregnancy, and if it exceeds this range, it is recommended to consult a doctor promptly.