Mismeasurements

I went in for an ultrasound and measured 6 weeks on day.. 1 week and 5 days went by so I was expecting to be 7 weeks 5 days. Instead my doctor told me I was 7 weeks 3 days. 😁😁😔 I've been freaking out. Why has (want a boy) he slowed down? This is my second pregnancy.. First ended in miscarriage  baby stopped growing at 10 weeks didn't find out till 12.. My question is it normal for the measurements to fluxuate?