Which is my correct due date??

Hey all so I'm a little confused. I'm currently what I thought was 23 weeks tomorrow but I just had my 4 week checkup and they said I was 24 weeks tomorrow. Now at my first ultrasound appointment I was supposed to be 7+5 but I measured 6+5 so I was then told my due date was August 10th going by the ultrasound so Iv been keeping track of my weeks by then but since my doctor told me I'll be 24 weeks instead tomorrow going by my last period my due date is actually August 3rd. My doctor recently left her practice and Iv had 2 different doctors since. So why did she tell me to go by the ultrasound reading and say I'm due August 10th when these 2 doctors are saying I'm due August 3rd. Which one is right the ultrasound or by my last period???? I'm just so confused...

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