Due date keeps changing?

My doctors office always confuses me. I got an ultrasound at 11 weeks 5 days on March 14th, they said my placenta was anterior. I got an NT scan and according to that ultrasound on the 14th i was suppose to be 12 weeks & 2 days. But they didnt update my due date & then the NT scan said posterior placenta & saying I’m 11 weeks 6 days! I’m so worried is fhe baby not growing everyone is saying everything is normal and we actually saw the baby move I’m so confused.