External genitalia differentiate during the late first trimester as the urogenital tubercle responds to dihydrotestosterone. Sonographic visualization depends on the angle of the genital tubercle and crown-rump length. Accurate identification requires specific morphological development of the phallus or labia. A. Week 12: Sonographic sex determination becomes reliable at this stage as the genital tubercle angle orients cranially for males or caudally for females. At 12 weeks, the phenotypic differentiation is sufficiently advanced for high-resolution imaging. This milestone correlates with the completion of early organogenesis. B. Week 10: While the bipotential gonad has begun differentiation, the external genitalia remain in a rudimentary, indistinguishable state. Ultrasound cannot reliably discern the small anatomical variations present at this gestation. Imaging at this stage frequently results in misidentification. C. Week 6: During this embryonic phase, the embryo is undergoing folding and initial neural tube closure. The primitive streak and urogenital ridge are forming, but external sexual characteristics are non-existent. Visualization is limited to the gestational sac and yolk sac. D. Week 8: The embryo enters the early fetal period with a bipotential phallus that appears identical in both sexes. Hormonal influences have not yet produced measurable physical changes detectable by standard obstetric transducers. Diagnostic accuracy for sex is impossible at this developmental point.