Sonography is a reliable method for the study of the morphological development of the external genitalia in fetuses 'in vivo'; it is possible to assign fetal gender in 95 to 99% starting at a BPD of 20 mm and to achieve an accuracy rate of 99 to 100% from a BPD of 22 mm, but fetal sex assignment should not be undertaken below a BPD of 22 mm, and especially not in cases where fetal sexing affects pregnancy management.