Fetal sex can be detected by USG at :

Male development is completed by 12 weeks and female development by 15 weeks By 28 weeks,the testis descends to the internal inguinal ring. By 36 weeks, one testis and by 38-40 weeks both testes descend into scrotum In female, the genital tubercle becones clitoris, the genital folds become the labia minora and the genital swellings became the labia majora Reference: Textbook of obsteics; sheila balakrishnan; 2nd edition; Page no: 67 and 475