What does biparietal diameter mean?

Biparietal diameter, abbreviated as BPD, also known as the fetal head biparietal diameter or the transverse cerebral diameter, refers to the distance between the widest portions of the two sides of the fetal head, specifically the straight-line distance from the outermost point of the skull on one side to the outermost point on the opposite side. It is one of the important indicators used in prenatal ultrasound examinations to assess fetal growth and development.

By measuring the biparietal diameter, doctors can estimate the gestational age of the fetus, predict fetal weight, and evaluate whether the fetus is suitable for vaginal delivery or whether a cesarean section or other delivery method is required. Additionally, changes in the biparietal diameter can help doctors monitor the health status of the fetus and promptly detect potential risks, such as fetal growth restriction or brain abnormalities.

The size of the biparietal diameter helps doctors assess fetal development. Normally, it should reach 9.3 cm or more at full term. As gestational age increases, the growth rate of the biparietal diameter gradually accelerates; for example, it is approximately 8.0 cm at 32 weeks of gestation and averages 9.28±0.50 cm at 40 weeks. If the fetal biparietal diameter is excessively large, exceeding the transverse outlet diameter of the maternal pelvis, it may increase the risk of dystocia.