20 Weeks Detailed Scan Not good

I just did my detailed scan yesterday, everything was OK except that was advised that my baby's heart is enlarged. Me and my hub are tested to have alpha Thalassaemia traits (we only knew during 19 wks), this could be one of the symptoms shown from the baby during scan. I've decided to do the amniocentesis test this week and the doc who counsel us told that based on the detailed scan, he is very sure my baby has inherit Thal. Major which is the worst case. There is hope for my baby as there is 3:4 chance of baby to be silent carrier/normal person but I don't know why he keep emphasised to test and to confirm baby is Thal. Major. Anyone went thru similar scenario when doc told u detailed scan result is not ideal but went thru further test and baby is healthy. I need some assurance that the doc may not be accurate as based on the scan as the amnio test result need at least 1-2wks. I don't wish to overthink and trying my best to be positive this period.