Hi mama! Honestly, better call Nova District directly kasi baka may specific requirements sila for CAS at 20 weeks, sometimes clinics have their own protocols na hindi same sa iba, at ang OB mo naman ang nag-refer so sigurado siyang may reason why 20 weeks specifically. A few of us mums dito ay nag-encounter din ng gaps like this between what our doctor wants versus what centers accept, so don't hesitate to ask kung pwedeng mag-accommodate or kung may alternative date na okay both sa clinic and sa OB mo. Goodluck mama, hoping they can help you out!