measuring 2 weeks ahead at almost 11 weeks

I am 38 years old as per my dates I'm 10 weeks 5 days and that's exactly how it has been showing on my scans until yesterday. When I went for the scan yesterday Doctor measured my baby's crown 12 weeks 5 days which is almost 2 weeks ahead. I asked him why? He said it could be possible that the measurement isn't accurate. And he refered me to nuchal transulancy tests. I'm worried if everything is ok. As my last pregnancy had to be terminated at 10 weeks as the Baby never developed a heartbeat and there was a fluid in the neck tissue.. Which is usuall found in Down syndrome cases.