My wife aged 27+, is almost 12 weeks pregnant. She had her last period on 7/4/19. Her 1st usg after pregnancy was done on 1/619. She is under regular check up of a gynecologist. He has given fol 123, sugest 300 sr and bevon. He advised to do nt scan on 7/ 7/19. But he did not advice double marker test and on 7/7/19 she will be on 13th week. Is it necessary to do double marker test and should it be done on 12 th week?