Cam doctors be wrong about how far along you are?

Ann
I went to the doctors on Wednesday and they couldn't find a heatbeat so they scheduled an ultrasound the next day. They found the heartbeat but said I was only 7w2d instead of 10w2d like I had originally thought and that's why they couldn't find the heartbeat with the doppler. But if I'm only 7 weeks then I would have found out at 3w4d that I was pregnant. This is my second pregnancy, first one was a blighted ovum, but the first time I didn't find out till I was 5 or 6 weeks along. Is it possible the doctors are wrong about how far along I am?