Checking For Down Syndrome

Hi, I'm a first-time inexperienced mum and has some questions. I am scheduled for a counselling and blood test session for down syndrome (optional test) on the 12th week and was told to go for scanning on the 13th week, similarly for down syndrome. Can someone explain why I need to go down to KKH twice for blood test and scanning to check for down syndrome? Would I be able to know the results immediately after the scanning? Is there anything that other experienced mums out there could share with me too, e.g. How is the blood test like? Many thanks in advance!