Although sonographic fetal sex determination is feasible in most pregnancies, in some cases, it may pose difficulties. An attempt to determine the fetal sex should not be made before 12-weeks' gestation because this early, it is relatively inaccurate. After 13 weeks, it is accurate in 99% to 100% of cases without malformed external genitalia. Sonographic fetal sex determination in the late second trimester is based on direct visualization of the external genitalia, whereas in the late first and early second trimester, it is based mainly on the direction of the genital tubercle (the "sagittal sign"): downward direction of the genital tubercle indicates a female fetus and upward direction a male fetus. Other sonographic landmarks, such as the fetal scrotum, the midline raphe of the penis, the labial lines, the uterus, the descended testis, and the direction and origin of the fetal micturition jet in males may contribute to the correct determination of fetal sex. Inaccurate fetal sex determination may occur when the external genitalia are malformed. Three-dimensional ultrasound, although of generally limited diagnostic value for fetal sex determination, may aid in better definition of congenital malformations of the external genitalia.