Fetal head anomaly Abnormal biparietal diameter 2025-11-15T12:26:43Z Abnormal BPD Abnormal biparietal diameter (BPD) refers to a fetal head width measurement, taken between the outer edge of the proximal parietal bone and the inner edge of the distal parietal bone on an ultrasound image, that lies outside the expected range for a given gestational age. The expected range typically falls between the 10th and 90th percentiles, with microcephaly defined as a BPD below the 3rd percentile (or more than 2 standard deviations below the mean) and macrocephaly defined as a BPD above the 97th percentile (or more than 2 standard deviations above the mean).