Triple marker test

Triple marker test kab hota hai,aur kya 1st ultrasound se pta lagaya ja sakta h ki mujhe triple marker test ki need hai ya nhi? aur kya ye compulsory hota h karwana sabhi moms to be ko?? Mujhe doc ne kuch v ni btaya iske bare me jabki mera 1st ultrasound hi hua h