A: BPD stands for biparietal diameter. It is a measurement of the baby’s head from side to side. The measurement is utilized to help determine the dating of the pregnancy, and is most accurate if done after the 13th week. As the pregnancy progresses and each baby grows at its own rate, the measurement becomes less accurate. In the third trimester, the BPD is helpful to check on the baby’s growth.

I often teach medical students a simple trick about the BPD measurement: if you multiply the BPD in centimeters by four, you will get a rough estimate of the baby’s age. For example, a BPD of 8.0 cm X 4 = 32 weeks.