Due date

Saquoia

I feel kinda dumb asking this but I'm young and a ftm...

I went to the doctor and the said my due date was July 20th.

I went to an ultrasound appoint and the measured baby and said I would be due on the 26th.

My question is, do I go off of my last period which was the due date my doctor told me, or do I go off what the ultrasound person told me?

Thanks in Advance! Means a lot