advice please x

Hello everyone. At 27 weeks & 4 dayside my 4d scan I'm measuring 2lb 6 oz. does this seem about right with everyone else? I know babies are different.... on my 20 week scan they told me 28th January for my due date & 4D scan told me 22nd January, how confusing!! Which one do I go off? And why has it changed? 22nd is no where near right with my last period xx where 28th is xx