Due date off 2 weeks?

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Hi ladies! My last period was March 2. I got my first positive April 3rd. I went in today for an ultrasound and was measuring 6.3 weeks. We saw the baby and heartbeat. My doctor assured me that seeing the heartbeat is great even though we have the dates off. My positive on April 3rd was not a squinted, it was clear as day. My blood work also put me at almost 9 weeks. Anyone experience this? I am still nervous even though we saw the baby.