Biparietal diameter (BPD), a medical term, also known as fetal head biparietal diameter, is the length of the widest part of the fetal head between the left and right sides. Generally after 12 weeks of gestation, the fetal skull forms a distinct halo, so the biparietal diameter measurement is performed from 12 weeks onwards. The biparietal diameter of a fetus at 13 weeks is usually around 2.0 cm, and it grows about 3 mm per week until 31 weeks, and 1.5 mm per week between 31 and 36 weeks. The biparietal diameter of a full-term fetus is usually between 9.0 and 10.0 cm. If the biparietal diameter of a full-term fetus is more than 10.0cm, the possibility of a huge baby should be considered.