Biparietal diameter (BPD) is a standard fetal biometry measurement taken at the level of the thalami and cavum septi pellucidi, measuring from the outer edge of the near parietal bone to the inner edge of the far parietal bone. BPD is used to estimate gestational age in the second trimester and to monitor fetal head growth. It is one of four measurements (along with HC, AC, and FL) used to calculate estimated fetal weight.