First ultrasound

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Hello mommas , today was my first prenatal visit. I was going in expecting to be 7 weeks 1 day but measured 6 weeks with heartbeat. Has anyone experienced this? My period is about 31 to 32 days in length. Last period was June 27th and i tested positive on July 26th which from a 6 week calculation would have put me at 3 weeks? I am to return in two weeks for a second ultrasound to see exactly when the due date will be. I’m a little anxious.