On an ultrasound, "BPD" stands for "biparietal diameter," according to BabyMed.com. It is the distance across the head from one side to another, much like the diameter of a circle in geometry.
At 13 weeks, the heads of most babies are tiny; the BPD is just 2.4 centimeters or about an inch. When the baby is ready to be born, the BPD is about 9.5 centimeters or about 3.75 inches. However, the size of the head can vary wildly, as some babies are significantly larger than others at birth, notes BabyMed.com. Other abbreviations used on ultrasounds are "EFW" for "estimated fetus weight" and "AC" for "abdominal circumference."